The sex of ducklings is not always obvious. Sometimes the color of the beak is a clue to the sex of the duck. This method is not always reliable.
In some species that feature brightly feathered males and drably feathered females, the sex of the duckling is pretty clear when the feathers come in.
When you can't tell by obvious feathers, there is the quack test. When very young, all ducklings make a squeaking sound. After several weeks, the voice changes. Females begin to quack. Males lack the voice box construction to make the quack sound, so they pretty much continue to squeak.
There is another way to determine the sex of ducklings. It involves grabbing the duck, turning it upside down, locating a certain opening on the lower abdomen and pressing your thumb and fingers around the area. Under this pressure, if the duckling is a boy, a body part will poke out of the hole. If nothing emerges, it's a girl.
Or you could wait a few more months and see if you can catch a gal in the act of laying an egg.