I have had Bantam Salmon Faverolles for three years now and I can tell for sure at around 3 weeks. The little boys will start to get tail feathers and the will be dark. You will see little pin feathers around the shoulders that appear very dark as well. The little girls will get the shoulder feathers a bit earlier, at least mine have and they are obviously rust colored. I usually wait until these shoulder or tail feathers start to come in. There is a subtle difference in the early wing feathers and unless you have one of each that are the same age, it is hard to tell without the comparison. My last two are little boys.