I was curious cause I have heard stories
Most Helpful Girl
I'm just going off me here, other girls are probably very different in their signs.
When I truly don't see a guy as someone I could be interested in and I know (assume) he's not interested in me, I just act like one of the guys (I am the only girl in my group of friends though, so being "one of the guys" comes naturally for me)
If it's a guy I don't know very well, or I know he's into me, then I still treat him like I would a friend, but I won't make eye-contact so frequently, or I'll act a little awkward I guess/try to avoid drawn-out conversations, and I generally won't agree to most plans/dates they try to make (I use the excuse "I'm tired/busy") until I know they don't see me in that way.
I know a few girls who will just ignore/avoid the guy altogether, and some who will just flat out say "it's not gonna happen, stop hitting on me".1