How to tell if a girl likes you or just being nice?

I really like this girl, but I don't know if she is just being nice. She just seems overly nice to me but doesn't know me that well. Now she and I just seem so comfortable to talk with each other. This has naturally progressed and it feels good. We know each other on first name basis. She is a lady that serves me coffee like once a week at this coffee shop.
How to tell if a girl likes you or just being nice?
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