You can tell by combs, when you combine them with the age of the bird. A mature hen should have a large, plump, red comb, but a young pullet under 4 months doesn't. Age is the key when using a comb to determine gender.
I'm not sure there's a difference in spacing between pubic bones when the birds are this young? Since a pullet of 8 weeks would not be laying yet, would her bones already be spreading? I'll have to check when I hatch some sex links.....