Good question - I'd like to know too - just hatched 6 last week - cute as can be. You'll love them.
My poultry books show turning them over and examining their little you know whats and see what pops out but I couldn't tell the difference - maybe I'll try again now that they are older and this time get my old lady glasses on so I can see.
Mrs. AKBB here... we just read up on that last night, because we have two Rouens and they're starting to look different. Our book says that in gray varieties, such as Calls, Mallards, and Rouens, the feathers of the crown of the head and back are often darker and have less brown penciling in drakes than in hens. The bill of a drake turns dull green while the hen's becomes dark brown and orange. But if there's any crossbreeding, it may be difficult to know for sure until the bird is fully mature.
There is a guy up the road from me that threw away a nice looking little house, vinyl siding, windows, little door. Looks like maybe a large dog house, I am going to stop and see if I can have it or buy it for a small fee. He just threw in his brush pile, Geez!!!! Don't people realize the possibilities??????
When I got my two Rouens they were already sexed at the hatchery, denoted by different colored legbands. BUT after I had the guy at the feed store cut them off we couldn't tel them apart. He said that the females are more noisy, like if you hold up a female chick it will peep more than the male will. It was true at the store, but now that they are 3 weeks old, they seem to be the same. Could be a regional thing too.