How to tell if a hamster is female or male?

Katie Asked: How to tell if a hamster is female or male?

Got a hamster like 6 month ago… the pet store said it was female. it looks female underneath by looking at pictures on google.. but sometimes when it's asleep and i pick it up it has two balls underneath but after about 5 seconds of picking it up the two balls go. I've named my hamster a girls and boys name so it doesnt really bother me i'm just curious for knowing. I asked the pet store do female hamsters have periods, they said yes but they arnt like dogs or humans ect it's just a bit of blood loss maybe like once ever couple of months for a day or two maybe.. My hamster has never had one, well what i can see off. My hamster is a syrian..