Actually it's really hard to judge whether the animal is seal or sea lion as they share many characteristics. But there are a few important features we can use to distinguish them.
Seals' (Phocidae) front flippers are generally furry, small and weak, so these marine mammals move on land by using their diminutive forward appendages to wiggle forward. They use their back flippers to generate powerful strokes when swimming in the water.
Sea Lion (Otariidae) have larger, hairless flippers which they use more effectively on land. Sea lions can rotate their hind flippers toward the ground so they're able to walk on all four appendages. They make use of their fore flippers when propelling themselves through water. They make use of their fore flippers when propelling themselves through water.
Detail information can be found HERE.
As I always run out of time, therefore I decided to come out a standard procedures. I got all the "shapes" ready beforehand, thought it might speed up the process. In fact it didn't work to me, probably because I prepared too many sea lions and pups (^_^).
This happy sea lion family consist of 5 members -- Papa, Mama, Koko, Meimei & Didi