Throw a ball and see if it has good instincts. Looks may help for sure but look into retriever puppy testing recommendations if it is a pup (search the web for some) and if it is grown see if it has the drive to retrieve.
There are so many Labs in shelters that you should be able to find one pretty easily if you are not looking at a pedigreed one from a known blood line. PM Belinda and she can put you on the right path there. Heck, hardly a week goes by on 2Cool that someone is not giving up a lab for personal reasons.