he will stare at you a lot. and when your eyes meet, he will either hold that gaze or look away quickly. he'll also talk to his friends about you. he'll also try to make you laugh. he will be nicer around you too.
He will listen to what you are saying. He'll try to make you laugh, and do nice things for you. He'll want you around, and call or text you frequently.
You can tell if a guy likes you. If he tries to start a subject with you and he tries to keep it going, even though its kinda lame, then he likes you. If he does wierd stuff around you or tries to make a smile, he likes you.
There are various ways to tell if a man likes you. Presumably he hasn't said outright "I like you". If he has, that is one way to tell. Another way to tell is if he is very stand-offish towards you. Usually this means that he is trying to hide something.
If a guy likes you he will become interested in some of the same things you are, talk to you more often and smile when he sees you. In addition he will ask your friends about you.