Okay but just know I have had alot of Nay Sayers on this but they just do the technique wrong and say it's inaccurate done right it works.
Take your chick and turn him over on his back so he is resting in the palm of your hand with his head between your thumb and index finger.
Hold his neck/head gently sometimes they will lay there without you holding their head now let him begin to relax as he relax's you will notice the chick will either stick out one leg or both.
You should get the results quickly I have had little roosters lay flat out like a board and hens stick out that one leg strait as can be
Hens will stick out only one leg doesn't matter which one
roosters will stick both legs strait out
Donot reflip or try to hold your hand on the chick trying to get the results you want as you will get a false outcome.
If the chick thrashes about put him down and try later.
I have tested this time and again and this gentleman picked me out 10 chicks out of a strait run bin at
TSC he said it was 5 hens and 5 roosters they turned out to be 6 hens and 4 roosters.