Telling the difference, male or female?

From Hamster Central WIKI

Its important to know the sex of your pet hamster, especially if you intend to breed or house same dwarf sex pairs together. Don't rely on a pet store to tell you the sex of your hamster, its best you do your own research and determine for yourself if you have a boy or a girl.

Below are examples of what to look for in a male and female Syrian hamster. Because of their size its fairly easy to tell the difference once they reach sexual maturity. Females are typically larger then male syrian hamsters and are said to show more personality then males, but of course that experience wildly differs from person to person. Both males and females would make wonderful pets.

In dwarf hamsters, telling the difference can be challenging due to their size! Dwarf hamsters are notorious for fooling even the most trained eye sometimes. Their small bodies and squirmy natures make them difficult to handle long enough to get a good peek at their parts. One recommended method is to place them in a clear plastic or glass container and looking at them from underneath. Be sure to make sure any handling is done in an area that is safe and also make sure your sitting down, so that your hamster doesn't take a bad tumble if it manages to squirm free.