Haha, I wish I had some magical way to figure out the sex of baby chicks! That would be great! I think that after 12 weeks, if there are no visible signs of a rooster (bright red enlarged comb and waddles) then you might just have a beautiful bunch of girlies!
These are the pictures I took when we first got our chicks, I had no idea what gender they were then. Now they are about 2 months old and it is VERY clear that we have a rooster, haha.