If the duckling is covered in down (fluff but no feathers) or is a juvenile (has mostly adult feathers but still may have down visible) and there is no visible parent nearby, look around thoroughly in nearby trees and on the ground for a parent. If the parent is present, monitor from a distance to ensure they reunite with the duckling.
If you find a duck or goose of any age that is:
Ducklings are most commonly found from March to August but can be found as late in the year as October. Mother ducks will not start incubation until all her eggs have been laid, if you find a nest of eggs, there is a chance the mother has not abandoned them and is instead just waiting to finish laying the rest of the eggs. Most ducks lay 8-13 eggs per a clutch.