Is she where you are a lot... by that I mean in your line of sight? She may not look at you but she will be aware of your presence and will glance sometimes when you aren't looking. She will have a hard and unaffected outer appearence but inside she is freaking out. Basically she will linger where you are hoping you'll talk to her or give her a sign you like her,
Depending how shy she is, she may not come near you at all, she's afraid of you so she'll have trouble standing near you. If you come nearby she may move or walk away if she's really really shy. If she smiles or blushes when you smile at her... if she looks away quickly that's the same thing as smiling. If she has trouble making eye contact sometimes in passing it can mean she's scared or uninterested... some may try to make eye contact on purpose but will have trouble smiling if you make eye contact...basically she will act cold or uninterested to the level of her shyness: Really shy = ice queen who appears to not care at all.
If you've talked already then she should be smiling. Her body language is open and she fiddles with something or touches her hair. Doesnt look away, maintains eye contact in convo. Seems genuinely interested in whatever you're saying. etc. Just ask her, she'll be nice about it. Shy girls are the sweetest usually. Don't let her go!