how to tell a male from female adult Crested ducks

[Goronson Chicks]

Curly tails and loud quacks are drakes and hens respectivly.

I have a group of 6 White Crested, one Golden Hybrid and one Khaki Campbell that I bought as 2 day old chicks on April 17. They are now almost 2 months old. Only one White Crested has a quack. The rest have a rasp. None have the curly tail. Which are they - drake or duck?