Here's a link to feathersite's Muscovie page, it may help a bit...

http://www.feathersite.com/Poultry/Ducks/Musc/BRKScobies.html

Velvet ~:>

About four to six weeks of age the males start growing faster. They are getting their juvenle (sp) feathers, and you can tell the diffence. Males are about 2/3's larger than the females when full grown.

Take your ducklings into a small area and look carefully at the feet, the feet of the males will be bigger.

I can pick them out because the males are clumsier. They trip more often and have a clunkier look.

I've sold all my young drakes already, I didn't have many this year. I'm keeping two young females, one because shes so friendly (a barred female) and a black female because she's so pretty, she's got a blue cast her to black feathers.

This is an old page, but it came up in the search so I'll comment, Muscovy drakes seem to be twice the size of the females, rather than 2/3rds larger.

I also hear that they have larger bills aswell