To answer your questions in order:
1) There is no real answer to this, sometimes the only way to see if it's a good idea is to throw yourself in the deep end and ask them out, other times it'll just happen gradually.
2) I had a long discussion with my entache about this sort of thing the other day, and basically we got down to the question
"How do you know if the relationship is a good thing, or if you've just convinced yourself that it's a good thing? And either way, does it really matter if you're both happy?"
We didn't come up with an answer to either question, but it makes you think. There's also the problem of (to quote so many tv programs) "being in love with the idea of being in love". How do you know?