There are two species of seal on the island; these are The Common Seal, which is not so common, and The Atlantic or Grey Seal, which is much more common.
The best way to decide which Seal is which, is to study the head, as The Common Seal has a much more doglike shape i.e. forehead and nose, whereas The Atlantic Seal has a very flat head. Common Seals also have very dark young, which they have in early Summer, whereas Atlantic Seals have white young in Autumn.
A golden rule when seal spotting on Mull, is that if you see seals basking on rocky islands just offshore, they are usually The Common Seal. If you are a few miles offshore and see the same, it is usually The Atlantic Seal.