Answer:
It really depends on the guy. I thought I knew a guy liked me once (which he totally did according to others) but he really only saw me as a good friend.
Some things I've seen guys do:
Talk to you a lot or make funny jokes.
It usually means he's trying to flaunt his social side.
Asks a lot of questions about you to friends, usually nonchalantly.
Ask your friends. 70% of the time he'll turn to them for advice on you.
Talks to you a lot on social networks
Facebook, MySpace, and Twitter hold A TON of info on your personality. He'll also use those for reference. if you can, you should run a scan to see if he's one of your biggest friends/fans.
Other than those things, most men are pretty clear with love, but don't get your hopes up just because he matches all these requirements. The best method is to flat out ASK--after all, what have you got to lose?