you can vent sex them the day(s) olds (if you are brave enough to attempt it). after that your next shot if voice sexing when they are umm, somewhere around 6+ weeks old. still dont know? time to look for drake feathers after they molt their teen feathers and get their nuptial plumage. HTH!
I had a go , but they seemed like they really didnt like it so i stopped and gave them big cuddles instead, guess I just gotta wait to find out now, I hope they are female though, then I can keep them both!