The comb. It's very large for the age of the chick (You didn't mention the age, but I'm guessing about three weeks by the size of the box) and in comparison to the others. I have had terrible luck with silkies, with every single one I've ever gotten turning out to be boys, so i have kind of...
The gray one is a cockerel. The others I'm not sure on. The one in front on the second pic looks like it could be a pullet for now. They are still a little too young to tell for sure.