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...

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...)