I have an idea. I think it's good. In fact, I think it's absolutely amazing! And everyone i've brought on board believes this as well.
Now here's my problem. What if I just explain the project well? How do I know if i'm really on to something great or if i'm just explaining my way towards absolute disaster and disappointment?
Ask everyone what problems they can think of. Look to historically similar things. Look to known human biases and see if you are falling to any of them. See if anyone is confident enough that they will put their money where their mouth is and bet on it succeeding if done as described. If you're still confident, build it ( note that lack of immediate success may not prove the idea is a bad one - maybe implementation or marketing).
You want to know if your model is a good model of reality then you need to test it against reality - but actually trying it might be the easiest way first, depending on what the idea is.