There are some ways to tell a seal from a sea lion. Sea lions have external ear flaps, seals don't. Sea lions can bend their hind flukes forward and "walk" on them, seals can't.
It's easy to tell which is the leopard seal when it and a sea lion are in the same image. The leopard seal will be the one on the outside. 83
(Well, unless they are friendly and mating, in which case it depends on the genders. But in general, the sea lion is the one that ends up as a bulge.)