How To Determine The Sex of 6 Week Old Chicks?

I have 5 buff brahma and 3 rhode island red bantams. They are now almost 6 weeks old. Is there a way that I can visually sex these chicks as I do not want any roosters? I am going to keep them in my backyard in a very neat coop that I have built. I have noticed that only one of these chicks is developing a comb that can be easily seen. Also, all of these chicks appear to be almost fully feathered except for two. I have read that roosters should be developing an obvious red comb at this age whereas a hen has a smaller more yellowish comb or no visible comb at all. I have also read that roosters are slower to feather out than a hen. I can provide pics if that will help.