You squeeze their little bum til it poops then if you see a bump its a boy no bump girl. I dont do that I am afraid I would hurt them. Best left to professionals. As for the eggs I tried that. No doesnt work. I got 50/50 on the hatch.
Vent sexing is only done on day-olds. And require a lot of training to not hurt the chick. There really isn't an accurate way to sex before hatching, despite the tons of old wives tales out there!