It can be a bit tricky til they get to about 4 or 5 weeks...then it's fairly obvious. Basically the boys have a larger gap between openings & the girls look a bit closer together. It isn't easy tho, and I just took home a 'female' kitten who turned out not to be, a couple of weeks later!!
It's very easy, even when they're only a day old. I have noticed with all of my kittens, the girls are almost always very flat in that area, but boys have large bump thingys. I don't know how to explain it better than that. Maybe my cats are just weird or something. lol Hope I was of some help.