Most guys I've know will prefer Drakes over hens,if for no other reason than the drake looks better with his plumage.
With some ducks it isn't possible to tell until after the bird is retrieved,like the Black duck.
We have large numbers of them here in Maine,large flocks of Blacks all season long.Even though we have lots of them we are limited to just one per day.Just as likely to bag a hen as a drake.Nothing you can do about it.
I don't mind settling for a hen Mallard but I look for the drakes.
My lab is still the best hunting partner there is.