Come on, we all see it! Just some things I've noticed:
- guy looks at girl while she isn't looking at him, she notices, and looks at the guy, the guy quickly takes his eyes off her... repeat (or vice versa) - pretty much what Lise said
- girl twirls / tosses her hair when talking to guy
- girl / guy is mumbling / stammering when they talk to the opposite sex
- girl / guy is blushing (DEAD GIVEAWAY!!! don't do this!!!)
- girl / guy stares at someone for an extended amount of time (too long = stalker.. people will not like this)
- girl / guy blinks faster when talking
- girl / guy touches themselves more (anxious, nervous, whatever) when talking
- girl / guy looks down when talking
Good luck with the book SD.