To sex a hamster you need to be able to look at its genitals. This can be done in one of two ways:
The concept of sexing any species of hamster is the same. The male is more elongated around the rear and more rounded than the female and this is particularly noticeable with Chinese Hamsters.
Looking at the genitals there are two 'vents' - on the male these are the penile opening and the anus, and on the female these are the vagina and the anus. The gap between these two vents on the male is usually 1-2cm apart (Dwarf-Syrian) and on a female the gap is so small that it is difficult to distinguish the two separate vents.