A lot depends on the type of guy and his individual personality.

Shy Guys
These are tough to crack sometimes...
He'll look at you, until you turn around, then boom, he's looking the other way.
You may 'feel' like he's watching you - but he's hard to catch at it
He may do something physical - like grab you in a play way, poke you, play with your hair...
(Remember when you were really young and the guy you liked punched you or tackled you?)
He may be working really hard to pay attention to you, but doesn't quite know how...
He may ask a friend of yours about you - he'll say it's just for "friend of mine" that wants to know about you.
He talks to everybody else - but when you're around he turns silent, or chokes up.
You seem to accidentally bump into him a lot of different places.
He may not say a word to you, but he shows up in the same line, at the same movie, etc.
He'll give you a little smile from across the room, but if you get near, he won't look up.
Basically - the BIG clue is that his behavior changes when you're around (compared to when he's around his buds or other girls).
DEAD Giveaway - when you talk to him he turns red.
(Bingo - you can pack that puppy up and take him home...)