The first evaluation takes place at 8 weeks, because before that time, tissues that hold the bones together are not fully developed. So, in general, at 8 weeks you see a miniature of the adult dog. At that time you check front and rear angulations, topline, balance, presence and temperament.
After 8 weeks the puppy grows unevenly. So its very difficult to guess, because everything goes out of place. Fronts and rears grow with different rates, so you just wait for everything to settle. Each dog and strain are different so the time that all will be in their place is hard to guess.
Also, yorkies are very hard to evaluate color wise. So...take your time and look at the signs.
I hope this link will be useful
evaluations
Its a written version of the old time classic "puppy-puzzle" by Pat Hastings.