We had a Chinese dwarf hamster a few years ago and when we bought 'it' at the pet shop we were told that when they are very young it is difficult to tell the sex. They said we could bring it back in about a month (I think) and when it was a little bigger they might be able to confirm the sex. At that point they thought our hamster was a female and since we were only getting one we just decided 'it' was a girl.

Calling the pet shop might be a good idea!

good luck