You can start by talking about your idea with your friends and see what they have to say about the idea. Then investigate whether there are similar products on the web and how well these products are doing. Finally launch an MVP and get as many friends as you can find to experience it and get their feedback. Try to post the MVP on the forums you frequent and pay attention to the feedback from users.

Have previously found sitting down with potential client and pre-release of software to be useful, particularly when allowing client to go "hands on".