What you DON'T want to see:
a. she looks distracted when facing you in conversation. If a woman is interested, she'll generally give you her undivided attention.
b. expressionless face...no animation in her movements. You bore her, move on.
c. poor (skittish) eye contact; that's her real coup de grace. You're toast, dude.
What you'd like to see:
a. She smiles and, ideally laughs alot when she's with you.
b. Her posture is mirroring you. She's directly facing you..if she's at an angle to you, and if that angle increases with time, she's literally letting you know that she's trying to get away from you.
c. When you let a lull in the conversation develop (always do that, or you run the risk of being a chatterbox bore), she picks up the conversational ball because she wants to spend more time with you.
P.S. always keep her wanting more by keeping conversations relatively short. End it before you really want to. If you overstay your welcome, she might think you're needy or a bore or that you don't have any friends or something. Chicks have this weird radar and they're real pros about interpersonal relationship stuff. Don't push your luck with them. Keep them wanting more, always.
d. she responds favorably to a suggestion that you meet up with her again. And you just know she's not feeding you a line.
That's my 2 cents.