i dont know about the mallards except that at some point when they get feathers they will look very different. but the pekins develop a drake tail around 4-6 months old. that was the only way i could tell. it's a feather that curls up and it is in their tail feathers. it's only one and you will notice it because it's the only one that curls. but it takes literally 4-6 months. either that or wait for them to lay eggs.
Know nothing about Pekins but I couldnt tell my mallards until they got the curl tail feathers AKA ( sex feathers ) and the heads turned green around 4 months old...Before that they looked female..They look female right now because they are molting and have lost the curl and green head..I can tell the boys from the girls now because the boys are alot bigger than the girls.....They are full grown but last spring they were all the same size.....
at 10 weeks it's deffinate for both, if it's a girl it'll be quacking, and if is a boy, it'll have a whisper quack type thing, much quieter, both of my new duckies are girlies, Jumper has been "quacking" for about a week now, and Junna has just started, they'll be a month old this thursday
by 10 weeks at the latest, pekin girls start to QUACK and the drakes keep a whispery chirp almost. at first, i just thought the girl was older and developing faster because i didn't notice a change from baby chirps to older drake chirps. i'm not sure about mallards.