How to know that Labrador is of good breed?

Well... A pure bred dog is not only a dog of only a single breed but also a dog that was bred for purpose through selective breeding. It's also a dog that is bred to breed standard.

Since you're getting this dog from "unknown source" the dog is not a genuine pure bred even if it is only Labrador.

You won't be able to tell and there's no test you can do. If the dog has webbed feet and an otter tail and looks like most labs then it's a lab... Even if it's not "pure bred"

Why are you buying a dog from an "unknown source" If you're buying it don't you know who you're buying from?

And by the way I have nothing against mixed breed dogs. I have a lab mongrel and he's a great dog. I do however disagree with buying dogs from people who breed indiscriminately.

If you choose to by a dog from an unknown source you won't know if the dog is pure bred or a cross bred dog.

Buy from a reputable, ethical breeder and you will get a pure bred dog with its papers. The Labrador breed club will direct you to an ethical breeder.

No, you won't be able to tell just by looking at the dog.

DO NOT BUY THAT DOG. The seller is probably an unscrupulous backyard breeder who will sell you a poor quality, sickly animal. You should only buy a puppy from breeders who are Kennel Club registered, and who allow you to examine the parents. Wait until after Xmas, and go down to your local animal shelter. You could get a puppy from there, but your best bet is to adopt a young dog (aged between 6 months and 2 years) because he would have already been housetrained and vaccinated.

look at lots and lots of pictures of labradors so you know. if it looks slightly different from the pictures it shouldnt really be a problem. do alot of research and do not buy dogs from kijii/craiglist, horrible place to buy dogs. also, make sure that you spend time with your dog before you buy it so you make sure it is the dog for you and does not have any behaviroul issues.

Check the breeder you are buying to make sure it is a reputable breeder.
Also make sure you get registration paperwork from breeder before you leave.
Ask to see both parents and their paperwork.
NEVER BUY FROM INTERNET, WITH OUT SEEING FIRST.

All I can add is that the most reputable breeders will have hips and elbows certified.

What do you mean an "unknown source". A breeder, a shelter, a random person?

You can not tell if a dog is purebred just based on looks alone. The only way you can tell is if the dog has a pedigree that goes back for several generations.

If the dog has no titles, no OFA, no CERF, was never shown then its not well bred. Since you are getting it from what is likely a BYBer and there is no way to know if its a cross breed. Its not a well bred dog

Read breed standard from wikipidia. A purebreed pup will have a fuller face and an otter tail and webbed feet.