If you go to a hobby shop and get some embroidery thread, buy two types (wool and silk). Then take a few strands of each. Now wrap them tightly around your finger. See how the wool one looks - broken and frayed? And the silk one is smooth and shiny. That is how to tell the difference with a yorkie's coat - to label the texture. An old mentor told me this and I have heard it ever since. It works pretty well to understand it. A wooly or cottony coat will have a wave in it too when it is just washed and you are blowing out the coat. You will see lines that go against the coat. I hope this helps.
It is so hard to tell for sure in photos. A silky coat will also feel and look like a satin ribbon - like girls wear in their hair when young. It will hang straight down, where a cotton coat is fuller and is puffy and won't shine as much if not at all.
I hope this has helped.
