Honestly, at 8 weeks old, if you are sure they are 8 weeks old, they all look like boys to me... However, if they are 14-16 weeks, girls can start to develop combs then. If you look at the pics that cjeanen put up, the BR roo is in the middle, and the girls are around the edges... where did you get these birds?
I would order 110% of the number of pullets you want from a place that sexes them. Reason I say 110% is because mistakes to happen in sexing, and finding homes for girls is usually pretty easy. Good luck with those boys there.