Most Helpful Girl
It depends what kind of personality the girl has. some girls may wear less to get your attention, and show some cleavage, but if a girl really likes you, I really don't think that would the case. Usually she makes an effort to talk to you, or laughs and smiles a lot, but both of these things could just be part of her personality.
Most Helpful Guy
Blushing, high pitch voice, sometimes though there won't be any of that and they actually give you every indication that they hate you instead.