According to FatmushroomHead: If she blinks, breathes or moves at all.
I'd say, she smiles, makes eye contact, her pupils enlarge, she might giggle, she might fiddle with her hair. She might do all of this if you are charming and she hasn't consciously thought "Wow i like him"
But i will admit, i'm really bad about giving signals that the older men around me misinterpret as interest. I can say over and over that im happily married, wear a wedding ring, talk about my husband and the second i look at him sincerely (this is what we ladies do to show we are listening to you) and BAM you get love notes in your locker with marriage proposals. It's all pretty confusing for both parties.
I mention the eye contact thing to my husband and Dad, both of them are like WHAT ARE YOU THINKING, looking a man in the eyes and not expecting him to think you want to have sex with him?! Ugh, i don't know, i look at Grandma the same way when she's talking to me too...i think.
Great Question DieselNuts (do you have the big rubber testicles hanging from your hitch?)