The method used in industry:
It is accurate in certain crosses, not any pure-breeding lines of chickens.
It uses the genes for fast-feathering and slow-feathering that are located on the Z sex chromosome. But it always requires a cross of fast-feathering father and slow-feathering mother to produce slow sons and fast daughters. (They might be the "same" breed, but they must be different in their genes for feathering speed, or it does not work.)
The details of the WING feathers might be accurate only up to a certain number of days, but feather-sexable chicks are easy to sort by gender for a month or so: the fast-feathering ones (females) grow a nice set of feathers, while the slow-feathering ones (males) continue to look almost naked for quite a few weeks.
For most breeds of chicken, both genders feather at the same speed (all fast, or all slow.)
But there are some people that insist they can sex other breeds of chickens by examining how the wing feathers develop, even if the chicks do not have different feathering-speed genes. I think that is what the OP here is trying to let people check.
I know what I'm talking about.
