With my recent hatchlings I have done the wing feather test, and it seems to work. Girls have two sets of feathers on their wings, whilst boys have just a line of feathers.
This can be done at about 2 days old and onwards.
There is loads of info about it to read up, also pictures and/or drawings which seem to illustrate it better. I did read it does not work for Pekins, thought, don't know why they are different, but I have used the method on all my latest chicks, and it does seem pretty infallable.
Used it to sex my RIRs, and have three gorgeous girls growing on, also on some more "Glorias" - Barred Plymouth rock, and it worked on them too.
the boys are now (not dealt with yet) very different from the girls in colouring.