|Afghan and Pug|
Researchers says they use visual cues.
Dogs have the largest morphological variety among all animal species. (If humans had as much size variation we would stand between 3 to 12 feet tall, and weigh between 30 to 500 pounds.)
|Two hundred pound Great Dane and three pound Yorkie|
They found that dogs form a visual category of dog faces. And they group those pictures of very different dogs into a single category, despite diversity. In other words, from Pug to Greyhound, Chihuahua to Great Dane, dogs immediately know a dog when they see one.
Scientific American has a really good article about the research.
Or read the journal article:
Visual discrimination of species in dogs, Animal Cognition, July 2013, Vol 16, Issue 4, pp 637-651