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Look for an open vs closed body posture. If she's squirming in her seat and angling herself away from you, she's creeped out. However, if she's relaxed and keeping her body posture open and facing you, she's probably a bit receptive. If she's keeping eye contact or making flirty eyes, she's probably receptive. However, if you see her occasionally looking towards the door, clock, or crowds of people, she's probably feeling trapped and looking for an escape. Break the tension by apologizing for bothering her and give her some space. She might very well relax enough to reenter the conversation. If she's giving short answers and seems to be trying to end the conversation then she's uncomfortable and you're probably not going to get anywhere. However, if she's giving longer answers or even asking you questions, then she's probably open to having a good conversation. If you see her flexing her hands, she's probably mad or offended that you're talking to her. If she's clutching her hands to her body or holding her purse tightly, she's at least uncomfortable. However, if her hands are relaxed and she's casually using them to talk, then she's more receptive to conversation. Sometimes a woman that seems receptive, friendly, or even flirty may quickly become closed and unsure. This is a natural emotional reaction to the course of the conversation and her own analysis of your interactions. If she's becoming uncomfortable, ease up. And remember: no matter how friendly or interested she seemed, it doesn't obligate her to give you her number or go on a date with you. Don't get angry if she says no.
Look at her body posture. See where her eyes are focused. Listen to what she's saying. Analyze what she's doing with her hands. Remember that signals can be mixed.