Here's a picture of the "underneath bits" of a male and female rattie for comparison

It's been noted that these rats look pretty uncomfortable. *G* Here are some more pictures for sexing younger rats:

Usually by one month of age, you will really know if you have a male though-- when the testes drop, they are half the size of marbles. Females will have a row of nipples down each side of their body, though they may not be visible through their fur.

If you were new to the world of rats, you will want to visit Virginia's Rat Page
a very comprehensive list of rat articles sorted by topic.