How to tell male from female hamsters?

You can tell male from female hamsters by their genitals. A male has a large, round rump with a obvious scent gland, a big long anus, and a large gap between the genitals and the anus. A female has a triangular rump, a small, tightly closed anus close to the genitals with a slit between them. Females also have teats on both sides of the belly, and male testicles become visible around 20 days old, especially obvious during the summer breeding season.