Girl always looks down when I walk past her, what's up with that?

A girl i have a crush on always looks down when i walk by her, i thought this was just her thing, but from what i noticed she never does that with anyone except for me, even when she's interacting with lamer, less attractive, meaner guys then me. The times we have talked she's seems to talk to me with a smile. How do i tell if i'm making her uncomfortable vs her liking me?
Girl always looks down when I walk past her, what's up with that?
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