And here is another diagram from http://www.pawsonline.info/sexinga.htm which is also quite helpful:
You’ll find that if you cannot determine the sex of your kitten from these pictures, then it is best to seek vet advice, or just wait a while until your kitty gets a week or so older. There are also a number of behavioural traits associated with both of the sexes, but these are not entirely reliable as each kitten has his or her own personality and a different upbringing which can affect these traits. Of course if you get your kitty from a breeder or an adoption agency then they should be able to tell you whether your new family member is a boy or a girl.