This little youtube video is the best distinguishing aid to tell what kind of coat your yorkie has. Of course the dog has to be a bit past the puppy coat stage to truly tell but this was the only thing that really explained it for me when I first got my yorkie. He had a thick coat that has since thinned out and turned to a delicate silky texture.
Silky coated yorkies look like drowned rats when water hits them while cotton coated yorkies fair much better.
https://youtu.be/SBVaiXw-bKY
You also must always take into account that a lot of the images we see of yorkies with these long flowing, silky coats are the result of intense grooming with products, flat ironing and all types of trick of the trade.