Haha! If you catch him snap a number of pics, there should be something that's telling. The drakes' eyes will also redden up pretty quick. Hens' eyes stay brown. The white on the face both juvenile hens and drakes get, though hens lose it and drakes keep it. The wing feathers are a good way to...
I can't tell on the wood duck. Can you get a pic of a wing stretched out by chance? The hens get the white cheek thing as juveniles too so that's not reliable to tell by.
The mandarins are correct.