Look at the Ears
This is the test biologists use, and it is why the two groups have the names they do. Sea lions and fur seals belong to the eared seals, the family Otariidae, and they have small external ear flaps you can see from a few feet away. True seals belong to Phocidae, the earless seals, and have nothing but a neat hole on each side of the head.
The flap is small, not much bigger than a thumbnail, so it helps to know you are looking for it. Once you do, it settles the question on any animal you can get a clear view of, and it works on a photograph just as well as in life.
How Each One Moves on Land
A sea lion can rotate its back flippers forward underneath its body and push itself up on long front flippers, so it walks, and it can move surprisingly fast across rock or sand. It can climb stairs, haul out onto a high jetty, and on a Californian pier it will shove its way through a crowd of its own kind without much ceremony.
A true seal cannot do any of that. Its back flippers point permanently backwards and are useless on land, and its front flippers are short and clawed. So a seal moves by flexing its whole body, a rolling caterpillar hump that looks awkward and is genuinely slow. Out of water a seal is nearly helpless, which is why they haul out on quiet beaches and ice.
The Noise Gives It Away
You will often hear a sea lion colony before you see it. Sea lions bark, constantly and loudly, and a crowded haul-out at a place like Pier 39 in San Francisco is audible for several streets. They are gregarious animals that pile on top of one another and argue about it.
True seals are comparatively quiet, producing grunts, snorts and the occasional slap of a flipper, and they tend to space themselves out rather than stack up. A beach of harbor seals is a row of separate animals, each one keeping a polite distance.
Underwater, the Roles Reverse
What a seal gives up on land it gets back in the water. True seals swim with powerful side-to-side sweeps of the back flippers, and they are the deeper, longer divers of the two. A Weddell seal can stay under for more than an hour, and an elephant seal routinely dives beyond 3,000 feet.
Sea lions swim with their long front flippers instead, rowing like a bird’s wingbeat, which makes them faster and far more agile in short bursts but less suited to the long deep dive. One is built to go down, the other to turn.
Walruses, and Where They Fit
There is a third branch, with exactly one living member. The walrus sits in its own family, Odobenidae, and borrows from both sides. It has no ear flaps, like a true seal, but it can rotate its back flippers forward and walk, like a sea lion.
Fur seals catch people out too. Despite the name they are eared seals, which puts them with the sea lions rather than the true seals. If it has ear flaps and walks, it is on the sea lion side of the family whatever its common name says.
