The pain of persuasion

I'm in a bit of a quandry - how do you know if an idea is a good idea before it has been implemented? When I explain my idea to people they all seem to think it's great. But is it a great idea, or did I project my desire for it to be a great idea onto them? I sell my idea hard because I really want it to be great, but sometimes I think that my ability to sell the idea is greater than the idea itself. And is that a bad thing?

Crap, I'm confused!