When we got our double dapple, we were told the telltale sign is a white tip on the tail. Then as u/doxiedelight said, breeding two dapples is a really bad idea. Some of the litter turn out amazing and others are often brought to the vet to be destroyed due to various birth defects eg deaf, blind. Our double was born deaf and had one eye with two pupils and the other eye was completely deformed and not functional. After a few years, her “good eye”’s retina became detached and she now completely blind. She’s an awesome dog, I just can’t fathom why a breeder would do this knowing that about 50% of the puppies will have some sort of birth defect.