They will look at you like they hit the lottery
She says, "Hey, I like you." despite what you may believe, sex doesn't mean you are dating or even on good terms with each other. Thus the feminine mystique.
if she smiles at you a lot, touches you, bakes you something or cooks you something, tell you you have pretty eyes, hair ect... laugh at your stupid jokes, listen to you talk about sports are other useless things.
she sniffs your crotch
she wont clip her toe nails in your presence
Ask her. Don't expect to read her mind in the absence of asking her. You can't.