How can you tell someone is flirting because they're genuinely into you and not just into the attention?

I'd talk to her about it. I would ask her what she feels for me and tell her I like her, but felt confused by the messages she was giving off. Tell her you noticed she would be shy one minute and then cocky the next and ask if you're just analyzing too much and she always feels a bit shy around you or if she is actually really cocky and occasionally puts up a shy act. Just talk about it with her and get it all figured out.