The common seal is smaller than the grey seal and has a different head shape. This is the easiest way to tell them apart: the common seal has a pronounced brow ridge, giving it a profile like a spaniel, whereas the grey has a distinctive long, straight nose, more like a wolfhound.
In Shetland both species have very variable coat colours and patterns, making that less of a distinguishing feature, but in general the older grey seal bulls are the darkest and largest. Some younger grey seal cows are very pale and can look almost white when dried out on a sunny day. The pattern of spots on the grey is more widely spaced than in the common, and the markings are larger. It can be surprisingly difficult to distinguish a young grey female from a half-grown common seal, however.
Grey seals tend to spend more time on the exposed, outer coast and common seals in more sheltered water but they often haul out alongside each other. Common seals bring out their pups in June, usually on secluded small islets, and greys usually pup in November, commonly on north-facing shingle beaches at the base of the cliffs. Common seal pups have very dark coats and can swim almost immediately, whereas the greys are born with a creamy-white baby coat and can't swim until they have shed it, when they are about three weeks old.