Size alone is not a good judge. I know many Silkies in my area that are 5-6 lbs. My Mylee barely tips the scale at 7 lbs and she is a Silky. however AKC standard is 9-11 lbs.
I personally think they have a distinct look to them. If you google Silky Terrier images
http://www.google.com/search?q=silky+terrier+images&hl=en&tbo=u&qscrl=1& rlz=1T4ADRA_enUS453US454&tbm=isch&source=univ&sa=X &ei=j5AJUf22BufuyAHYzIGYDg&sqi=2&ved=0CC0QsAQ&biw= 1920&bih=852
I think the Silkies stand out pretty clearly even though there are some obvious Yorkies mixed into these pictures the Silky characteristics become easier to identify when you can see many to compare to at the same time.