I have found, as a general rule, if they obviously look like a boy, they are, if they obviously look like a girl they are, and if they look like they could be either... my experience is that they are girls. Every time I haven't been sure, it has turned out to be a girl. Of course, this is only over two litters (the first litter the girls were easy to spot- they were tortie) and so it isn't an especially tried and tested rule, but it's about the best I'm going to get at the moment! So what I'm going to do is make a reasonable guess, and name the kittens on that basis. If I am wrong, the names will change and I will look a bit of a wally, but I'm used to that!
You can see quite clearly here the anal opening, testicles and the prepuce which contains the penis- kitten penii are retracted permanently until about six months, I believe, a fact which I am sure no-one will find remotely useful ever! There's quite a long distance between the dot of the anus and the dot of the penis and the prepuce is about the same size as the testicles.