Good luck trying to sex them. I gave up on my two Rouens and had to wait until they both started laying in order to feel 100% confident what their sexes were LOL Both of them were quite vocal but had different sounding quacks. Then I read that you can start seeing a little green sheen in the head at some point, checked both of my ducks out and only one of them seemed to have the sheen in the sunlight. So I figured it was a boy. Then that one developed a darker bill...and the one I thought was a girl got a really orange bill. The "girl" also started to get a white collar while the other developed a white spot. That's when I gave up.
Now they've both started laying so I have two girls. One is quieter than the other, one has a really dark greenish "female" bill and the other still has a really orange bill (even though Rouen females have darker bills?) and a neck collar like a boy would have. So I'm not really sure what's up with that. I can at least rest easy now though. haha