By reading your face. And someone spent lots of $ trying to determine this? Just ask any dog owner and they could tell you without all the fuss. Anyhow, read the story @ http://time.com/3708391/dogs-mad/ and this: Dog walks nearly 20 blocks to see owner with cancer in hospital @ http://www.usatoday.com/story/news/.../dog-walks-20-blocks-owner-hospital/23345157/