I've heard of a new technique been pioneered on some of the BIG US game farms, for sexing day old ring necks. Its supposed to give accuracy of 85% + dependent on the experience of the person, I've only heard of what they do so I'm a little fuzzy about what the differences look like, basically there is apparently a noticeable difference in the size and shape of the wattles of the face in chicks, if you compare male to female (I know its small in chicks and it only exists as a fleshy frill around the very edge of the eye however there is apparently a noticeable difference! ) It would be worth experimenting with some rings and seeing the results I think.
I know when I bred reeves they could be sexed very accurately extremely early on by the size of the developing spur bud in the chicks........
Regards