Our medium sized girl Millie was a rescue pup and we have always wondered about her lineage. She is very quiet and calm, about 45 lbs. Appropriately, she has a ? on her side.
By educated guess I assume that would require someone who was qualified to judge dog breeds and was expert in the individual characteristics particular to certain breeds. Very few of us have access to that level of knowledge. I would expect the swag of a dna check has to be better than a wag. Pound puppies may be many generations removed from any purebred strain.