Well most you can tell by looking at them and/or their parents. There are so many that it is almost impossible to get papers
How Do U Know If U Have A Full Blooded Lab?
If that's what you are looking for, you purchase your puppy from responsible, ethical, knowledgeable, breeder who is AKC (or similar, not a pet registry) registered. In many cases the sire at a minimum (often the dam as well) will be DNA tested and if theres something questionable the pup can be DNA matched to the sire. The best place to start looking is at the breed parent club. It's not a guarantee their great but it's the best place to start and you'll learn what to look for in a breeder. There is no other reliable way to tell for sure. Otherwise consider rescue.
www.thelabradorclub.com www.wildheirlabradorrescue.org www.petfinder.com
www.thelabradorclub.com www.wildheirlabradorrescue.org www.petfinder.com