These are tough to crack sometimes...
1) He'll look at you, until you turn around, then boom, he's looking the other way.
2) You may 'feel' like he's watching you - but he's hard to catch at it
3) He may be working really hard to pay attention to you, but doesn't quite know how...
4) 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.
5) He talks to everybody else - but when you're around he turns silent, or chokes up.
6) You seem to accidentally bump into him a lot of different places.
7) He may not say a word to you, but he shows up in the same line, at the same movie, etc.
8) He'll give you a little smile from across the room, but if you get near, he won't look up.
9) Basically - the BIG clue is that his behavior
changes when you're around (compared to when he's around his buds or other girls).
10) DEAD Giveaway - when you talk to him he turns red.