I personally judge by wattles. Comb size works too, but I seem personally way more accurate judging wattles mostly if not completely and the comb only if necessaryAfter they're dead, I can be certain: cut them open and look inside for testicles.
I prefer to start with the one who has the biggest comb and work my way down by comb size, checking each one after killing it. Often I can be 100% correct, but I never have more than one oops (because if I find a pullet, I know not to do any more.)