First of all, read Lean Startup by Eric Ries. That will get you started if you actually do it.

Secondly, the specific technique that might work in your case is to invest a trivial amount of money using ad words or some other advertising vehicle to produce an ad for your non-existent product. Have a landing page for the ad that describes the product. Harvest email addresses of people who are interested in applying for access to your "beta" on that page. That will help you gauge whether or not people are actively looking for a solution to the problem you think you could solve.

Expect to iterate.