I think you can tell if she is checking you out if she looks away suddenly and blushes if you catch her looking at you.
But if you want to get to know her don't sit around and wait for her to come to you. Get to know her. Become friends and things may progress into more than that. If not, atleast you made a new friend.
try and make your eye contact last for just that little bit longer than normal.. but don't stare at her all the time or she would think you are a freak
I agree with Sloe Moe. Just don't sit on your hands. Go up and introduce yourself. Take the chance that you will get embarressed and shot down. Maybe you will be lucky and she has been checking you out. Or, she hasn;t been and is now really impressed with your huge balls because you had the courage to talk to a complete stranger. Or, she wont be interested. 2 out of three odds isn't too bad! And a little rejection isn;t all that bad.
Just go with your gut.. odds are.. if you think she is checking you out.. she probably is. Even if she isn't, she will after you approach her. Girls really aren't all that mean.. usually.. take a chance, you never know what might happen.
I never had any idea when anyone was checking me out till one time the woman hitting on me madeit a little obvious, and almost got into a fight with my g/f at the time.
but thier are signs, the looking away is an easy one, also suble things like when she's talking to you, she may tease her hair, adjust her clothes (uncontiously trying to make herself look more pleasing to you) or if she gives you a smile and wink, you in.