Pekin ducks, also known as Long Island ducks, are a domesticated variety of the common mallard and perhaps the most familiar farm duck in the United States. Donald Duck was in fact based on the Pekin duck. Unlike wild mallards, male and female Pekin ducks don't have distinctive differences in plumage -- they are all white. They do have enough other physical and behavioural differences, however, that distinguishing the genders is straightforward.
Look at the bird's tail. Male Pekin ducks, drakes, have a single, curled-over tail feather together with numerous straight ones. This curled feather is absent in females, which have uniformly straight tails.
Listen to the bird. Female Pekin ducks, hens, are much nosier and make loud quacks. Drakes have weak voices.
Look at the back of the bird's neck. A hen may have some missing feathers there, removed by a drake when he mounted her.
Ducklings are very difficult to sex, even for experts. Sexing ducklings involves examining their anal vents, which is stressful for the bird. Unless you have an urgent reason for needing to know the gender of your ducklings, it is best to let them mature, when the differences become obvious.