In your signature, is Nismo the fifth dog over? He looks a bit like a Yorkie...unless you clipped his coat for that picture? Unless a dog is a proven champion, you can't really know whether or not they are true to the breed standard.
Yes Nismo is the fifth dog over. If he looks like a yorkie, well then that a crazy looking Yorkie. In that Picture Nismo had just got out of the lake. Maybe that isn't a very good pic but he sure isn't no yorkie.
For your fact above there are always DNA testing to find out of there is any other breeds in the background or if someone is lying about the backgrounds, etc.