Ask HN: Simple idea to generate revenue

It seems like the panacea for many developers is to come up with a simple idea that can be monetised effectively. There are a few success stories (e.g. Buffer), but it seems that there are even more ideas that crash and burn.

What I find really difficult is to determine whether or not an idea is going to be a success quickly. Sure you can develop a 'lean' prototype that you release and iterate on, but then how do you know if that concept failed because it was too incomplete and you stopped the development too early?