How do you know if your idea for your new business is viable, meaning it has a realistic chance of being successful.
Well, you can use the techniques of Idea Validation to investigate your idea. Passing the tests of idea validation in no way guarantees the success of your idea but it at least if done well increases your odds for success.
But first you need to have a viable startup idea. And Resource (1) below, How To Get Startup Ideas by Paul Graham, Nov 12, 2012, is a really good place to start. What follows are excerpts [but be sure to read the whole article as his examples are critical]:
The way to get startup ideas is not to try to think of startup ideas. It’s to look for problems, preferably problems you have yourself.
The very best startup ideas tend to have three things in common: they’re something the founders themselves want, that they themselves can build, and that few others realize are worth doing. Microsoft, Apple, Yahoo, Google, and Facebook all began this way.
Why is it so important to work on a problem you have? Among other things, it ensures the problem really exists. It sounds obvious to say you should only work on problems that exist. And yet by far the most common mistake startups make is to solve problems no one has.
When a startup launches, there have to be at least some users who really need what they’re making—not just people who could see themselves using it one day, but who want it urgently. Usually this initial group of users is small, for the simple reason that if there were something that large numbers of people urgently needed and that could be built with the amount of effort a startup usually puts into a version one, it would probably already exist. Which means you have to compromise on one dimension: you can either build something a large number of people want a small amount, or something a small number of people want a large amount. Choose the latter. Not all ideas of that type are good startup ideas, but nearly all good startup ideas are of that type.
When you have an idea for a startup, ask yourself: who wants this right now? Who wants this so much that they’ll use it even when it’s a crappy version one made by a two-person startup they’ve never heard of? If you can’t answer that, the idea is probably bad. 
How do you tell whether there’s a path out of an idea? How do you tell whether something is the germ of a giant company, or just a niche product? Often you can’t.
So if you can’t predict whether there’s a path out of an idea, how do you choose between ideas? [The author’s answer is unsatisfying–basically, hope you are the kind of person with good hunches.]
If you’re at the leading edge of a field that’s changing fast, when you have a hunch that something is worth doing, you’re more likely to be right.
The verb you want to be using with respect to startup ideas is not “think up” but “notice.” At YC we call ideas that grow naturally out of the founders’ own experiences “organic” startup ideas. The most successful startups almost all begin this way.
That may not have been what you wanted to hear. You may have expected recipes for coming up with startup ideas, and instead I’m telling you that the key is to have a mind that’s prepared in the right way. But disappointing though it may be, this is the truth. And it is a recipe of a sort, just one that in the worst case takes a year rather than a weekend.
Once you’re living in the future in some respect, the way to notice startup ideas is to look for things that seem to be missing. If you’re really at the leading edge of a rapidly changing field, there will be things that are obviously missing.
Most things that are missing will take some time to see. You almost have to trick yourself into seeing the ideas around you.
But you know the ideas are out there. This is not one of those problems where there might not be an answer. It’s impossibly unlikely that this is the exact moment when technological progress stops. You can be sure people are going to build things in the next few years that will make you think “What did I do before x?”
And when these problems get solved, they will probably seem flamingly obvious in retrospect. What you need to do is turn off the filters that usually prevent you from seeing them. The most powerful is simply taking the current state of the world for granted. Even the most radically open-minded of us mostly do that. You couldn’t get from your bed to the front door if you stopped to question everything.
Pay particular attention to things that chafe you.
When you find the right sort of problem, you should probably be able to describe it as obvious, at least to you.
Which means, strangely enough, that coming up with startup ideas is a question of seeing the obvious. That suggests how weird this process is: you’re trying to see things that are obvious, and yet that you hadn’t seen.
Give yourself some time. You have a lot of control over the rate at which you turn yours into a prepared mind, but you have less control over the stimuli that spark ideas when they hit it.
Give yourself some time. You have a lot of control over the rate at which you turn yours into a prepared mind, but you have less control over the stimuli that spark ideas when they hit it. If Bill Gates and Paul Allen had constrained themselves to come up with a startup idea in one month, what if they’d chosen a month before the Altair appeared? They probably would have worked on a less promising idea.
A good way to trick yourself into noticing ideas is to work on projects that seem like they’d be cool. If you do that, you’ll naturally tend to build things that are missing. It wouldn’t seem as interesting to build something that already existed.
Just as trying to think up startup ideas tends to produce bad ones, working on things that could be dismissed as “toys” often produces good ones. When something is described as a toy, that means it has everything an idea needs except being important. It’s cool; users love it; it just doesn’t matter. But if you’re living in the future and you build something cool that users love, it may matter more than outsiders think.
So if you’re a CS major and you want to start a startup, instead of taking a class on entrepreneurship you’re better off taking a class on, say, genetics. Or better still, go work for a biotech company. CS majors normally get summer jobs at computer hardware or software companies. But if you want to find startup ideas, you might do better to get a summer job in some unrelated field. 
Or don’t take any extra classes, and just build things.
Because a good idea should seem obvious, when you have one you’ll tend to feel that you’re late. Don’t let that deter you. Worrying that you’re late is one of the signs of a good idea. Ten minutes of searching the web will usually settle the question. Even if you find someone else working on the same thing, you’re probably not too late. It’s exceptionally rare for startups to be killed by competitors—so rare that you can almost discount the possibility. So unless you discover a competitor with the sort of lock-in that would prevent users from choosing you, don’t discard the idea.
If you’re uncertain, ask users. The question of whether you’re too late is subsumed by the question of whether anyone urgently needs what you plan to make. If you have something that no competitor does and that some subset of users urgently need, you have a beachhead. 
The question then is whether that beachhead is big enough. Or more importantly, who’s in it: if the beachhead consists of people doing something lots more people will be doing in the future, then it’s probably big enough no matter how small it is.
Err on the side of doing things where you’ll face competitors. Inexperienced founders usually give competitors more credit than they deserve. Whether you succeed depends far more on you than on your competitors. So better a good idea with competitors than a bad one without.
You don’t need to worry about entering a “crowded market” so long as you have a thesis about what everyone else in it is overlooking. In fact that’s a very promising starting point. Google was that type of idea. Your thesis has to be more precise than “we’re going to make an x that doesn’t suck” though. You have to be able to phrase it in terms of something the incumbents are overlooking.
A crowded market is actually a good sign, because it means both that there’s demand and that none of the existing solutions are good enough. A startup can’t hope to enter a market that’s obviously big and yet in which they have no competitors. So any startup that succeeds is either going to be entering a market with existing competitors, but armed with some secret weapon that will get them all the users (like Google), or entering a market that looks small but which will turn out to be big (like Microsoft). 
There are two more filters you’ll need to turn off if you want to notice startup ideas: the unsexy filter and the schlep filter.
The unsexy filter is similar to the schlep filter, except it keeps you from working on problems you despise rather than ones you fear.
There are great startup ideas lying around unexploited right under our noses. One reason we don’t see them is a phenomenon I call schlep blindness. Schlep was originally a Yiddish word but has passed into general use in the US. It means a tedious, unpleasant task.*
*This paragraph is from the author’s post Schlep Blindness, January 2012. The post is very brief and powerful so click on the link and read it and then get back to the excerpts. Basically, Schelp Blindness causes us to unconsciously avoid start up ideas that will involve a number of tedious unpleasant tasks. The author uses the example of the online payment solution Stripe.
Resuming excerpts from Resource (1):
Turning off the schlep filter is more important than turning off the unsexy filter, because the schlep filter is more likely to be an illusion. And even to the degree it isn’t, it’s a worse form of self-indulgence. Starting a successful startup is going to be fairly laborious no matter what. Even if the product doesn’t entail a lot of schleps, you’ll still have plenty dealing with investors, hiring and firing people, and so on. So if there’s some idea you think would be cool but you’re kept away from by fear of the schleps involved, don’t worry: any sufficiently good idea will have as many.
While the best way to discover startup ideas is to become the sort of person who has them and then build whatever interests you, sometimes you don’t have that luxury. Sometimes you need an idea now. For example, if you’re working on a startup and your initial idea turns out to be bad.
For the rest of this essay I’ll talk about tricks for coming up with startup ideas on demand. [The previous part of the author’s post deals with what he terms the organic method of generating start-up ideas.]
When searching for ideas, look in areas where you have some expertise. If you’re a database expert, don’t build a chat app for teenagers (unless you’re also a teenager). Maybe it’s a good idea, but you can’t trust your judgment about that, so ignore it. There have to be other ideas that involve databases, and whose quality you can judge.
One good trick is to ask yourself whether in your previous job you ever found yourself saying “Why doesn’t someone make x? If someone made x we’d buy it in a second.” If you can think of any x people said that about, you probably have an idea. You know there’s demand, and people don’t say that about things that are impossible to build.
The next best thing to an unmet need of your own is an unmet need of someone else. Try talking to everyone you can about the gaps they find in the world. What’s missing? What would they like to do that they can’t? What’s tedious or annoying, particularly in their work? Let the conversation get general; don’t be trying too hard to find startup ideas. You’re just looking for something to spark a thought. Maybe you’ll notice a problem they didn’t consciously realize they had, because you know how to solve it.
When you find an unmet need that isn’t your own, it may be somewhat blurry at first. The person who needs something may not know exactly what they need. In that case I often recommend that founders act like consultants—that they do what they’d do if they’d been retained to solve the problems of this one user. People’s problems are similar enough that nearly all the code you write this way will be reusable, and whatever isn’t will be a small price to start out certain that you’ve reached the bottom of the well. 
(1) How To Get Startup Ideas by Paul Graham, Nov 12, 2012.
(2) Schlep Blindness by Paul Graham, January 2012.
Test Your Idea With These Simple and Inexpensive Steps by Adam Callinan, June 18, 2014 from Entrepeneur.
Validate Your Business Idea — Quickly — With These 5 Steps by Peter Gasca, September 18, 2014, from Entrepeneur.
9 Entrepreneurs Reveal How They Validated Their Business Idea by Anand Srinivasan, April 29, 2014 from Entrepeneur.