How can you tell if a guy likes you?

I think this guy likes me but I'm not really sure. I get tat feeling tat he likes me. He is always starring at me. we both looked at eachother in the eyes & they way he was looking at me was unexplainable. I think I saw a twinkle in his eyes. I was talking to this guy more than once & he saw it & i think he got jealous cuz he imed me online and asked me if i liked the guy tat i was talking to. & he is always sending me ims online first. I dont know wat i should do cuz i like him back. Should i drop more hints till he admits himself he likes me? or shoul i jus go up to him & ask him? plz help