You lean towards her and she leans back away from you.
She starts turning away from you.
She doesn't make eye contact with you.
She folds her arms across her chest.
Double crosses her legs at the ankle.
She touches her nose often while she's with you.
A limp or hanging hand can indicate boredom with you.
When talking to you, her eyes keep glancing at other guys around her.
She makes no effort to talk to you. She doesn't reply to any of your comments, doesn't answer your questions, and doesn't ask any questions about yourself. Unless she just doesn't feel good or is moody this could indicate that she's not interested in you and you will be wasting your time.
And last, she just come out an tell you to get lost, go away, go to hell, or ever worse, to fuck off!