Eh? The only indisputable sure sign would be that he tells you he likes you and/or tries to initiate a relationship. There's other "signs" but they can have other meanings as well. A few not-definite-but-possibly signs...
arrow He goes out of his way to hang out with you/do nice things for you/etc.
arrow He makes a big thing of pointing out things he's noticed about you, does for you, etc.
arrow He frequently initiates physical contact with you--touching your arm, giving you a hug, ruffling your hair, etc.
arrow Body language cue--if you're standing together and talking, his feet and hips face you.
arrow He asks if you like anyone/are going out with anyone/etc.--yeah, sometimes it's just curiosity as a friend and sometimes he's asking on another friend's behalf, but if he asks frequently and seems stressed by vague answers, it's possibly his roundabout way of telling you he likes you.
arrow He pays attention to details of what you like, what you need, etc.
These are all indefinite, since any of them could have other meanings (he's just being friendly, he's the touchy type, he's just a pervert, he's got a friend who likes you, he's observant by nature) but put enough of them in one case and it's a significant possibility that he likes you. But all around, the only way to know for sure is to ask. Give it a shot--what have you got to lose?