It's hard to tell until they are in their adult plumage.....but by now it should either have a really loud quack...or more of a monotone rasp. Girls have loud quacks, boys have raspy quacks
Definitely the quacks. Girl quakes loudly, boy is soft. I like to say he follows her around as she’s quaking loudly saying quietly, “yes dear, yes dear”
Also, the curly tail feathers that flip up. Different color bills...I’m trying to remember ...but one is definitely greener.