There are more lab mixes than there are stars in the sky. The majority are lovely dogs. I've often said, "You can mix a lab with almost anything and get a fabulous dog." Lab/Golden retriever mixes are pretty common.
I've had two labs. I was never 100% sure the first one was all lab, but he was the best dog I've ever had. The second seemed a likely lab/weim mix, though I'll never know for sure. I will always regret that I never did a DNA test on either of them.
What will you do if it's not a purebred? Are you planning on showing or breeding? If not, it likely doesn't matter, unless you're paying a great deal of money.