You should check his feet. If they are webbed he is pure.
Notice the tail. It should not curl over the back, or be 'feathered'.
Please research purebred labradors, English and American, before adopting. Go to a good quality breeder. Just because it's a purebred doesn't mean it's a quality dog. English labs range from $700 and up. American start $400 and up. Don't go to a breeder that over breeds a dog, is a puppy mill, or a fly by nighter. They breed for money only and not for love of the breed, show, or want to make another family as happy as they are. Most good breeders have the pups born and raised in their homes with the rest of the family. You may also go to www.akc.org or a quality breeder website such as Four Seasons Labradors, Endless Mountain Labradors, Ricochet Labradors, etc. They give a lot of information on what a quality purebred should look like.