We used to say that the reason most startups die is they built something nobody wanted. Now, startups build things people say they want, but don't actually buy when it comes down to it.

How can you predict this? What out for the following.

They don't describe the need that they have the same way you do.
They don't have the budget, support or resources ready now.
They haven't done anything about it, like trying stuff or researching solutions.
They're not real people you've met - they're just figments of your imagination.