I don't sex my baby chicks bc I don't want to injure them--they are SO little-- (chickens, not ducks), BUT I do butcher my birds, so this may help. Look for the male sex organ, which is a whitish spherical organ about the size of a large marble, located adjacent to the vent, but upper, not lower and inside of the vent. This is where the rooster's have theirs. Your ducks are old enough to examine without hurting them. I don't know ducks, but I'd be surprised if their anatomy differs much from chickens.
I agree about the Mallards--males have a green hood and, I think, a ring separating the other feathers. I know what you mean about BYC. Every minute the recent posts resets for a new group of questions. However, go to your profile and set it so that you'll be notified of answers to threads you have started or posting on. This will arrive on your email, and give you a shortcut link to that thread. Hope this helps, from a Forum Addict, here and (right now) on Horse Forum.com **ducks4you is embarrassed to ADMIT this addiction**