There are lots of ways to prove that people want something. The best is (obviously) to launch something lovable. ;-) We proved it by launching a "coming soon

There are lots of ways to prove that people want something. The best is (obviously) to launch something lovable. ;-) We proved it by launching a "coming soon" page with a form to submit your email addy to hear about the launch. We had a 25%+ conversion rate on that form and got thousands of email addys. Another way to prove it is to show 3 or 4 apps that do something similar that SUCK, but people use them/pay for them anyway. And there's plain ol' market research.