Brown leghorn roos will be very obvious by 6 weeks. Their combs and wattles should be quite large and red, and their coloring will start to look different than the pullets. That is if they're single comb birds. Brown leghorns also come in rose comb. Which is yours?
I'm looking through pics, and will see if I can find a pic of my brown leghorn at that age, or close to it.
Mine have much bigger and redder combs, I have no Leghorn pullets to compare them to, they were part of a mixed dozen I picked out (supposed to have all been pullets). Thanks for the pics!