About my chocolate lab puppy is he purebred or mixed?

I just purchased a chocolate lab puppy last month from a farm in PA. The owners told me he was purebred. But looking at him now at 4 and a half months of age I noticed that his tail is not otter like which is a characteristic of labs. His tail is more thin than fat. But it is straight and not curled and towards the end of his tail he has swirly hair. It is too early to tell or is he not a purebred after all. I really want to know because if he is not a purebred I want to get my money back.