The males there is room for testicles. So as kittens there is a large space that develops. I guess its pretty easy to tell with males if they are fixed. For females I think its a little harder. The stomach sags a little when they are spayed and the vet might be able to feel to tell if they are intact or not.
Meeka was originally thought to be a male. Sometimes as kittens its really hard to tell. After a while if no testicles show then you know its a female which is what happened with Meeka otherwise the breeder would have kept my girl.
The best way to show the difference is probably a diagram vs. trying to explain it:http://www.pawsonline.info/sexinga.htm