Personally I would start off by assuming that she forgot to give me the change back, and I would try reminding her by asking something like "How much did the beers cost?". If that didn't have any effect I would probably ask outright for the change back.
By now a red flag would be going up in my mind and I would be watching her for any further deceptive behaviour. With her being a female and all, I would probably find a shitload of manipulative and deceptive behaviour going on. I would probably quietly brood for a little until she does something so blatant that I can't contain myself any further. I would confront her about it and she would probably start off by playing coy and ignorant.
As I press her on the issue and want to see some acknowledgement of her wrong doing she will most likely start getting all mad and try to turn it around on me by calling me "petty", or dig up something about me that she doesn't like in hopes of deflecting the issue. Women don't get far with me when it comes to changing the subject and trying to confuse things. Once again, seeing as she's female and all, she will absolutely, positively not admit that she's wrong...because women are never wrong. At this point things are extremly damaged to me and I will never see her the same way again.
That in a nutshell is pretty much every relationship that I've ever had with a woman.