I can tell because the females always have the longest wing feathers and the males and way shorter ones.
I have heard that also , Wish it worked for silkies but they feather in so different than regular feathered birds, I hope it does work for D' uclles as I was using that method also and if so I got 2 pullets and one roo. I know you can do that on the cochins and on cochins the tail feathers on the males don't grow as fast as females. I will have to google and do some research.