A young man recently told me he'd noticed that you could pretty much tell which girls will have sex with you based on how they said hello.
If they light up when they see you, and greet you with a drawn out "hellooooo," it's pretty much a done deal.
If they give you a perky "Oh hi, how are you" or the like, it means they won't have sex with you. (Politeness is the kiss of death.)
But strangely, if they are somewhat brusque, and just said "Hey," or even downright rude, and just say, "Oh," the odds go up again. This is either because these are girls who are shy around you because they're attracted to you, or because they're the types don't care as much about social propriety.