Like I posted above, feather sexing is accurate for sexlinked crosses with one breed being fast feathering and the other slow.Good Question. Some breeds are easy to wing sex shortly after hatching. There are two rows of feathers on the wing edge. One with staggered or uneven in length would be a female while those with mostly even (in length) would be males. It doesn't work for all breeds.
There is a lot of mis information out there! While being fun to do, it is not reliable for selling since the error is very large for gender.