How to tell if a girls annoyed?

I see this girl at school. I like her (obviously) and today she was acting weird. I would catch her trying to look at me but usually with a blank face (no smile, no frown, just whatever) but she would try to catch my eyes. She woul look at me thru the corner of her eye. I don't know what it is. I think she is annoyed. I kinda sense it. So just please tell me what are the signs of an annoyed girl? And is she (the girl in the question) annoyed? Thanks