There are several ways to tell the sex of a duck, but it may depend on the type of duck. The feathers provide the best way to determine the sex, but they may not develop until the duckling is slightly older.
For a mallard breed, the females are brownish and gray, while the males have a rich brown color on their chests, green on their heads, and a white ring around their neck. Male long-tailed ducks have grey feathers around their eyes, while the females have white feathers around the eye area. Male ducks may also have feathers that curl at the base of their tail, and are usually larger than females.