Sexing can be a little difficult but incase of breeding you have to find out. Flip your hamster on it's back gently.
This method is best to do when your hamster knows you quite well.
Males- At the bottom of his stomach there will be two holes about a centimeter apart and his bottom
is a bit long. Also, at a certain age he will have quite noticable testicles.
Females- Females also have two holes but they are very close together and they may even look
like one. She will also have four pairs of nipples along her stomach. On many females, you probably won't be able to find
her nipples so looking at the holes is best.
Can't flip over your hamster? Try putting him in a clear box or jar (no need to close the lid, if it's small just place
a hand lightly on top), and look at the bottom of it. This should also work.