A silky is a breed that came from crossing Australian Terriers to Yorkshire Terriers. So, it is often difficult to tell them apart. A silky averages 10 pounds where Yorkies are supposed to be under 7 pounds -- but we all see Yorkies over standard and there are probably silkies that are under their average too. The silkies have a single coat like a Yorkie but it does not grow to floor length. They have a greater variation in the blue, silvery blue being okay -- where the standard for Yorkies is to remain steel blue. But again, we all have seen plenty of Yorkies that lighten to a silvery color too. I have to say I would not always be able to tell the difference. I have seen AKC registered Yorkies that I thought fit the Silky standard better. I have not seen a registered Silky up close but the pictures are very close.
Here is the Silky Club of America si and it does have a page about telling the difference between the three breeds:
http://silkyterrierclubofamerica.org/