It's a boy, err, uhh... You won't be able to tell a tortoise's sex until it reaches a certain size, which varies by breed. The most obvious tell is the plastron—for mating purposes, it's flatter on females and curved on males. Males also tend to be larger and have longer tails.
If you're a tortoise owner who prefers surprises, just wait for your pet to come out of his or her shell. Males will eventually display their private parts while soaking. And it's not uncommon for females to lay eggs, even without a mate to fertilize them.