When they’re 8-10 weeks pick them up, and the females will have a hard quack sound and the males will sound more raspy. The older they are the more accurate it is if you’ve never done it. You can look up some videos of people doing it too to hear the sounds. The males will also develop curly feathers on their tails when they get older. You won’t be able to tell by their plumage for quite a while.