How do I tell if my new pet rat is male or female?

Question
I am an inexperienced rat owner, and I just recently bought a rat. The lady that sold me the rat was a very experienced rat owner, and she said that it was a female, but I looked at her the next day, and saw a round pink thing right under her tail that looked every bit the male part. So, I had my Dad look at her/him, and he is also convinced the Bitsy is a male. Can you tell me for sure?

Answer
A male baby, even if sucking his testicles in, will have a little protruding penis not quite halfway up his belly. When he decides to let his testicles descend, you'll see two pink, oval shapes just under the base of his tail.

A female baby lacks testicles (obviously), and has a smaller bump, her vulva, closer to her anus than a male's penis is (much closer).