My co-founder often says, "the idea of a lifetime comes along every fifteen minutes." I bet most entrepreneurs feel the same way.
What was it about your current venture or startup that led you to believe it was the "right" one? What motivated you to take this opportunity, as opposed to all the others?
EDIT: In hindsight, after you have paying customers, it's easy to tell if your idea works. But what motivated you to try one particular idea over all the others open to you? Was it just a gut feeling "just do it" sort of thing or was there something particular that caught your interest?
Pre-launch/revenue, I think the best way to figure out if it's the "right" idea is answering honestly whether or not you'll use the product yourself, and if you're charging for your product, whether you'd pay for it. Don't answer this as a hypothetical "you" in an alternative universe -- ask whether you would buy this product today to solve specific problems you currently have.
This is a well established pattern: 37signals created Basecamp so they could manage their projects, FreshBooks was created so they could invoice their clients, MailChimp was created so they could send email campaigns for their clients, and I created FormSpring because I had to build online forms for my clients.
Talk to as many colleagues, clients, friends and family that fit your target market to get a sense for whether or not the problem you're trying to solve actually exists. (Focusing on the problem in these conversations is probably more fruitful because those close to you are apt to try to make you feel good by saying your idea is the best thing they've ever heard, even if it sucks).
Then launch a fake AdWords campaign to a) see if the market for your product exists and b) gauge how hard it will be to generate leads/sales. See Eric Ries' excellent post about this for more info.
The only thing that "proves" it's right is when other people give you money for it.
Seek "that which produces revenue." When I started my idea was only barely good enough to make money, but by making money it opened the conversations that led to the truly good idea that made millions of dollars. It's my story and that of many other companies.
So get an idea that's "good enough," get out there, and listen to what others are telling you.
I thought I had the perfect idea, 2 years later I realized that the idea I was working on brought me to a much better idea. When it all comes down to it don't worry too much about the idea, it's mostly about execution and how you do it. Also don't be afraid to change that idea if it leads you somewhere else.
Like Ade I am a big believer in eating your own dog food (will you gain significant value using this product yourself) and Jason, get started knowing that what you may end up with may be completely different. Having worked on a number of projects over the years I also learned a lot about looking for popular ideas that were still being done in old ways.
Our current company came about because of all of the above reasons:
So we made the connection and created a solution for the startup, SMB, SME market for implementing strategic plans in an online environment focused on action items that would walk people through the execution of the plan.
In the end we have used our own software to launch the company and the product and we are constantly using that experience to make improvements in the software. We have even gone a step further and documented via our blog several key aspects of our plan and shared that with our users and readers.
So in summary the right idea for us to focus on was one where believed there was already a well documented market that was still behind the times in software offerings, one that nearly every company in existence would need at some time or another and one where we had direct experience in using the product to grow our own business.
I'll post my experience here. I decided to start up my own company while working for another firm. They were having trouble expanding their software product to serve the needs of a different customer base. Eventually they gave up on the idea -- but I didn't.
I realized that there was a market need for new software application that could help those customers. And I thought that I and my co-founder might have the skills to create that product.
So in one sentence, I found a problem I knew I could solve better than anyone else.
That's what made me start my own company.
If thousands of people are willing to pay for your product, you know you it's a good idea.