If she has a frown or a scowl on her face. If she is super good looking. If we have heard gossip about her and what we have heard is bad. If she has earbuds in her ears. If she is being a bitch to someone in front of our eyes (guys watch how girls behave).
She smiles. She looks friendly. She does something nice for someone in front of us. She looks happy and laughs a lot. She flirts with the guy. She smiles or smirks with the guy.
I never approach anymore, i've given up chasing, im lazy. Even if a girl smiles at me I just ignore her and continue my thing. Why don't you just approach him instead? Most guys would be happy to be approached. If a girl approaches me then she has my attention, cause I know that takes confidence. And confidence in a woman is rare, and sexy.