"Distinguishing this skua from the northern hemisphere Arctic, pomarine and long-tailed skuas is relatively straightforward. The large size, massive barrel chest, and white wing flashes of this bird are distinctive even at a distance. The flight is direct and powerful. Identification of this skua is more complicated when it is necessary to distinguish it from the closely related great skua of the North Atlantic, and the other large southern hemisphere skuas. Identification problems make claims of any southern hemisphere skua in the eastern North Atlantic problematic, and few records of south polar skua have been accepted in Western Europe. Similar problems occur, of course, with extra-limital claims of great skua."
However, my Skua sightings have been on Patteson pelagic trips and Debbie Shearwater pelagic trips so I don't have to do much identification as they know these birds like the back of their hands.
This first one in NC on a Patteson pelagic off Hatteras.
This molting one is from a Shearwater pelagic off Monterrey, CA.