Forget about the idea. What matters is the people. In priority order:

1. Do you trust them in the "If I died they would make sure that my family was taken care of" way?

2. Are the executing right now even without technical talent?

3. Do they have a track record in the industry they are about to try to enter?

That's it.

Now the red flags:

  Do you really want to work with *two* idea guys?

  Do you really want to work with two people who think
  4 or 5 hours a week is going to get a solid app built?

  Do you really want to work with two people who believe
  that high pressure is a good way to establish a long term
  business partnership?

Go with your gut. This won't be your last offer to work on someone else's idea. Don't get sucked into the idea of owning a company. Particularly when you don't have a controlling interest and there is an aggregate controlling interest that sees you as providing commodity value.

Good luck.