Two of the more flashy drakes in the waterfowl world are the wood duck and the Chinese mandarin. The hens of these two species actually look quite a bit alike. Here in the Rockies woodies are not very common, and mandarins are scarce (they’re not native although there is a small population of free-flying, self-sustaining birds in the region).
I’m not sure if anyone gives a cul de canard, but here’s an example of each:
The wood duck hen typically has a white eye mask shaped like a tear drop. The bill is slightly longer, and normally has a black nail (the tip of the bill).

Mandarin hens have a round, white eye mask which extends as a line towards the back of the head, but the eye will sit higher than the line as opposed to being centered. The bill nail is light in color, and the bill is slightly shorter. Female mandarins also tend to have white streaking on the auriculars (cheeks). Wood duck hens may have auriculars lighter in color than their crown, but rarely will white appear.

I’ve mistaken these two hens a few times, and happened to see one of each on the same day recently.