Maybe watch "her" waddling around and have a look at the back end
Females have a kind of rounded back end. Males have a sort of extra oval shape at the end (over the testicle area).
You can sort of see it from the side on this photo
http://www.hamsterclub-germany.com/p...0SH%20Male.JPG
It's particularly noticeable on Chinese hamster males, so that might help - Vectis's page on sexing chinese hamsters shows some good photos of a male back end
Sexing Chinese Hamsters
Female butts are much more rounded
Male
http://knowyourmeme.com/photos/761457-hamster-butts
Female
Hamster Butts | Know Your Meme
First pic on this link gives an idea.
Bandeds