This photo gives you a good look at bill sizes and shapes on these two species.

Compare the slope of the forehead on the Cackling Goose in the foreground and the Lesser "interior" Canada in the rear. The Cackling's forehead is much steeper and jumps up from the small bill where the Canada's slopes away, the Cackling's flat head compared to round head of "interior" Also note the obvious pale edging to the mantle feathers on Cackling Goose.

When relaxed, the black stocking of the neck on the Cackling is about as long as it is wide unlike the elongated stocking on the Lesser.