I can't till around a month, I tell by the curl feathers(When they get them) personalities, who leads them around, how big or smaller, markings (Some of my pekins have had a black spot on his beak) And I pay close to different features, like markings on the beak.. sometimes they're scratches and how thick, small the feet are. Also by how noisy, I have an all drake flock with one hen mallard, she is always quacking her head off but my smaller pekin with a much curlier tail feather will quack with her, whenever she does, the other one won't be as loud and noisy. Not sure if this is helpful but it sure has helped me.