Well, there are several ways to sex ducks. First, there is vent sexing. That can only be done when ducklings are a day old, and only by professional sexers, so that isn't really a method good for beginners.
Another difference between the sexes, noticeable as they get older, is differences in voice. Female ducks are noisy. They quack and quack often. On the other hand, the male ducks (drakes) are silent. If they make noise at all, it is a raspier, quieter sound than produced by a female duck.
When your ducks are mature, in most cases, you can tell their sex by the presence of sex feathers. Those are a few curled, stiff feathers that only appear on a drake's tail--female ducks don't have them. However, sex feathers can take a while to grow in. There is one breed of duck that does not have sex feathers--the Muscovy.
For many breeds/varieties of ducks, the drakes are often a different color than the females, too. They have brighter plumage, glossier feathers, and a distinct pattern. Usually, the female ducks are drabber in appearance. Keep in mind, though, that while they are young (before they grow in their adult plumage), both male and female ducks look the same. The drakes will also change their appearance into female form each year when they go into their ecclipse molt.
A last way to tell gender with ducks can only be used with Muscovys: size. In that breed, the males are much larger and more masculine in appearance than the female ducks are.
As you can see, there are several ways to sex your adorable ducklings. However, you probably won't be able to tell their genders until they are at least 8 weeks old, or are developing their adult plumage.