You can determine the gender of kitten by looking at the urethral opening of your kittens. The male's urethral opening is round whereas the female's opening is slit.
The way the vet explained this to me was to look 'down there' and if it looks like an 'outy' belly button he's male, if it looks like an 'inny' its a female! After one tragic mistake (named the kitten Jazmin and he ended up being just Jaz) I was taught this trick and have been right every time since!