It's really hard at that age, we would guess with ours, but were normally wrong. You can sex them, but I was told you can really seriously hurt them if you do it wrong. Best bet is to wait for their tail feathers to come in, if they are curly, its a drake. Also, male ducks sound like grumpy old men, where females quack. Hope this helps... eventually!
I was gonna say muscovy don't get tail feathers nor do they quack, but @Miss Lydia beat me to it!

Females trill and have soft quiet noises unless broody or yelling at each other... males get huge and hiss or huff a lot


