This may sound a bit disgusting, but you must look in their private area. If it is a girl. you may see small nipples along the stomach. if it is a boy, you will see a penis in it's private area. I...
A male rat has testicles and a female does not! Take a peek under the tail! They are rather noticeable! If the male is neutered it may be more difficult. In that case compare the rat to another rat...
Boy rats have large testicles at the base of their tail, it will be obvious because the testes are not covered with fur. The sacs are about the size of a peanut when the rat is fully grown, but will...