This is one of the questions that I see a ton of, and there’s really no 100% way to tell. The most precise measurement to tell whether or not your new Rottweiler dog or puppy is purebred is to have proof of his bloodlines and parentage, like his pedigree. A puppy that is purebred will have registration papers that are authentic from a reputable and recognized breed registry like the British Kennel Club, Canadian Kennel Club, American Kennel Club, or any other club or registry that is uniquely devoted to that certain breed.
The papers will confirm that the parents of the puppy are purebred, as well as the generations that went before them. Don’t ever buy a puppy that is said to be purebred if there are no papers that go along with it.
The second way to tell that your dog is purebred that is reasonably reliable is DNA testing. It is a science that craft very accurate and specific results, but it depends on the dog breeds being tested, and there are some skewed results and some room for error.
However, in some instances, DNA tests can get incredibly pinpointed. For instance, if you have a Rottweiler puppy who has American Kennel Club papers, for instance, and you need to be sure that his sire is the dog that is listed on the pedigree, you could be able to get certain confirmation of that. This is due to the fact that the AKC necessitates a DNA profile to be listed for dogs that sire multiple litters, so if your American Kennel Club pup has a good sire, and you have doubts, you can rest assured tat you will know whether or not it is true.
There are loopholes in every system, of course, and unscrupulous and creative breeders can, and do, often exploit them. There is no water-tight, 100% guarantee. If you take your time, and pick your breeder thoughtfully, you will get a much higher chance of acquiring that purebred pup that has the bloodlines that you need.