How can you tell if your shih tzu is a pure bred?

Hi, to tell if a shih tzu is purebred is by looking at it's face, legs, body, and head. The head is round. The face is flat. You can tell if it's flat by how the nose is flat. Where the mouth is, it should look like there is circle around it. You check the legs and see if it is short. If it is, good. Now you look at the body. It must be roundish. It must look chubby. The last thing to look at is the tail. If the tail is pretty long like 8 inches and is not curled up, that's a mixed shih tzu with another dog probably with a Japanese chin. The tail must be kind of short and curled up. If your shih tzu has all of these features, your shih tzu is pure bred.