Paying close attention to a girl's body language is a great way to tell if she likes you. Be on the lookout for a girl who crosses her legs toward you and plays with her hair while holding your gaze. While you don't want to have a weird staring contest, eye contact is important; it shows you have confidence, which is extremely attractive.
Most girls know what they're doing when it comes to flirting. If the light squeeze she gave your arm made your heart pump double-time, it was no accident. More than likely, she's interested in you and is trying to let you know in a subtle way, hoping you'll get the message and return the gesture.
If a girl folds her arms while talking to you, doesn't smile and rolls her eyes when you try to tell her a joke, it might be time to move on to another available fish in the sea. A girl who's interested in what you have to say will listen to you and give you her undivided attention. But one who has other things on her mind will allow her eyes to wander, paying attention to everything that's going on except for her conversation with you.
Still not sure if her body language is giving you the green light? See how a girl responds to you taking her hand if you want her to follow you through a tight crowd. If she squeezes your hand back, this is definitely a positive sign. If she drops your hand like a hot potato, try not to take it personally, but this shows she's uncomfortable and might not be into you.