Free: Contests & Raffles.
purebread or not.. Does it matter?.. Dog likes your home... What would decide you keeping the dog?...
I would have him scanned. If he is as nice as you say he might be microchipped .No sense letting the family fall in love before you know for sure.
I look for birdie labs that aren't to high strung. Some pure breed field trial dogs are to much for me. You only need the papers if you plan on showing/ field trials or breeding. I have had some great dogs that weren't pure breed. Extra branch in the family tree helped out. I wouldn't worry as longs lab doesn't have pit or rot in him/her.
Labs vary in color. Just because it has a spot or off colored hair doesn't mean it's not pure. Look at a black bear with a white chest patch. Does the patch mean its momma hooked up with a polar bear....No. Same goes for labs. All it means is that it won't be a show dog. Has no bearing on it being pure.
Should have a slick outter coat along with the softer unlayer thereby having the double coat.
I would also post a lost and found ad on your local paper, radio, or craigslist. There might be a family that is heartbroken out there somewhere.