Most species of hamster are quite easy to sex. Hamster males have quite prominent testes which are your first indicator of their sex. But, when they’re young, long haired, and a tiny robo, you may find yourself scratching your head over whether your hamster is male or female.
In general, females have two nipple lines from the underarm to the groin. The nipples may be somewhat hairless, but will be flat. Females have three vents - urethra, vulva, and anus. These holes are quite close together. The shape of the tail then resembles a triangle.
Syrian / Dwarf / Chinese / Robo Females
Contrarily, males usually have very prominent testes. These are quite obvious on Syrians and Chinese hamsters especially. The testes tend to stay tucked in the scrotum unless they are feeling warm or comfortable. The testes themselves are round, hairless and smooth. Males have two openings - the urethra and anus. The testes are in the middle, between where the urethra and anus are, making quite a large gap between these areas. On a male Syrian, the anus is almost directly under the tail.
Male dwarfs also have a very prominent round scent gland in the middle of their belly. This can help you distinguish the sex as well.
Syrian / Dwarf / Chinese / Robo Males