How do you know if a guy has a crush on you?

So how do you know if a genuine nice guy is interested in you for a relationship and not just a fling.
What will he do?
What will he say?
How will he act?
What should a girl who shows interest back do if he doesn't take the initiative to ask her out?

If you're talking about a certain someone you're unsure about, please explain further.
People are different, and some are very good at hiding their feelings, so you never know whether they like you or not.