[philco]

Talk to people, ASAP. Hone an elevator pitch, go to a coffee shop, and pitch strangers the idea in exchange for buying them a cup of coffee.

Tell them it's your "crazy cousin's" idea, and that you don't think he should quit his full time job for it, but you wanted to see what other people thought of it before you got back to him. (This will make it so that they'll be more honest with you. If they know it's your idea...they'll feel bad telling you it sucks).

Are people giving you their business cards and asking you to email them when it launches? That's the litmus test, make it so that people want what you're pitching so bad that they GOTTA HAVE IT TODAY.

Otherwise, don't even bother building it.