You could get all scientific and try to see if it has external ear lobes. Then you know its a seal.
But I go by size: if it's big, its a sea lion. If it's 11 feet long and 1,200 pounds (2 - 3x's a Sumo wrestler's size): it's a male sea lion. Eight feet, 500 pounds: it's a female sea lion. A seal is 200 pounds as an adult. Needless to say it is really easy to determine which it is when you are kayaking and one of the two comes up to you. When your heart suddenly starts racing, you know your adrenaline has kicked in because your subconcious noticed a sea lion before you could even meaure it's size. Anything that is almost as long as your boat and 3 times as wide is something that makes you pray!
Sea lions swim under your boat and seem to say, "You are lucky that I am in a good mood today because I could easily bump your toy boat and have my way with you". Seals come up to you with their puppy dog eyes and seem to say, "Want to play?" I hear they have come on shore to play with some people's dogs. I keep taking my dogs there but I guess they don't look friendly to the seals.
Do you get which I like better? There are a few people in Kodiak who are known to have pieces missing from their buttocks. Sea lions like to jump on board the fishing boats when they are unloading their catch. If you try to push them away from your catch, they are likely to push you over the side. Or in a few cases, they'll take a bite out of you. In fact, seals are thought to be related to nice small otters and sea lions are considered related to bears. They say if you find a sea lion skull near on a beach you might have a tough time determining it is not a bear's.
Anyway, here is a video I shot of a sea lion rookery this summer. They seem nice and playful when they are that far from you. Enjoy them at a distance.
Notice in the video how the sea lions are able to push themselves and almost "walk" on their hind legs. I hear they have remnants of hip sockets that let them do that. This is why they can climb such steep cliff walls. Seal are more like fish; they lack those hips. When on land they use their front paws like paddle wheels on a Mississippi riverboat to pull themselves forward. No steep beaches for them.
Enough for today. Another day we'll talk more about their first few day of life and perhaps about what I've learned form people who used to hunt them.