It's a difficult one. They should come through when they get their second set of feathers i.e. their adult fathers (Fluffy down -> Juvenile feathers -> Adult feathers).
It's a bit dodgy though because they may not come through , or they can fall out/get pulled out for some reason! I know both my drakes are missing theirs at the moment. I think one has lost them because they keep having scraps and his drake feathers have suffered from it and the other one didn't seem to grow them back when he last moulted!
By now you should be able to voice sex them. That's a more clear, definite way of telling if they're male or female - loud, honking quack is female, raspy, squawking quack is male