Snow Geese (the one in front) have a black line along the gap in their bills. They're also a tad bigger than the Ross's Geese (the two in the back here) and have a somewhat more sloped head. Ross's, besides being smaller and having a more spherical head shape, have shorter bills and tend to have knobbly bits on the back edge of their bills.
Oh, and I'd seem some sort of white goose at a park by San Francisco Bay some years back, but too far away to be sure which, so I've counted both these species as lifers this trip.
