It helps if both parents have a silky coat but like you said that is not always a sure thing. A 12 week old puppy that is going to have a silky coat I think would still have a black head or it might be breaking to a dark gray. I think if you are seeing a lot of tan on the head at 12 weeks then there is a good chance the coat will not be silky. Some of the cutest puppies have the full fluffy, thick coats at 12 weeks with lots of leg furnishings, these types of coats usually do not usually turn out to be silky. Most dogs with silky coats usually have a thinner coat as a puppy. Of course there are always exception and also there are many coat variations and it is very difficult to predict the end results on a 12 week old puppy unless you are very familiar with the lines behind the parents.
Here is a sticky that talks about puppy coats and other things.
http://www.yorkietalk.com/forums/gen...rt-thread.html