Here's a useful guide for beginning bird enthusiasts. Waterfowl, depending on the location, season, and circumstances may occur as individuals or in mixed flocks. Usually we first see them in flight and at a distance, and species-specific characteristics are not discernible.
The first step in any serious attempt to identify birds is to refer to a field guide in advance and learn what birds to expect in the area for the season. Next, in the field, we generally try to determine whether the bird we see in flight is, for instance, a duck or a merganser, or a goose or a swan. Often the major groups of waterfowl can be identified by body shape and flight characteristics. With patience and further observation, the bird is likely to tell you what species it is.
This graphic is one in a series I developed while teaching in the Department of Forestry and Natural Resources at Purdue University (1975-1992).