The pro's can sex them at day olds, they check the chicks vent thats how they tell?
auto sexing breeds like cream legbars you can see at day olds and sex linked are the same.
Some watch the wing feathers grow and sex that way but thats at 1-2 weeks I believe.
I use the needle and cotten methord it works for me but gives all a laugh.
or wait for wattles to appear4-6 weeks, what breed have you got?