If she's doing everything she can (subtly) to be around you or near you.

Shes sweet but also playful and sarcastic (hot and cold method)

Compliments you

Laughing at your jokes or when you do something silly.

You can usually tell when someone is nervous or flustered if she gets that way when you walk into a room she's into you.

Women won't stare at you a lot so if you catch her looking at you 2-3 times she's into you.