ID'ing these birds in flight is a much greater challenge in the field, for obvious reasons. But the same features still apply, and are relatively easy to see in a photo. For real-life applications though, try to notice several field marks, the most useful of which is probably the longer neck of Snow Geese. In a mixed flock, the smaller size of a Ross's Goose will often jump out at you as well. In this photo, you can clearly see the detail of the size and shape of the bill, and the grin patch. Bill shape should be used with caution, however, as the angle of a flying bird can make a bill look deceptively short in flight.