Good question! So what's the answer? It's simple - height and weight, right?
Rule of thumb for buyers to gauge by-
Height: A puppy will grow at least 3 inches more from the 8 week old mark until maturity. Meaning a puppy that is 5 inches high at 8 weeks will be at least 8 inches at adulthood. If you want a small (not so tall ) teacup make sure it isn't more than 5 inches tall at 8 weeks. A true measurement can be gotten with a carpenters T-ruler. The puppy must be measured from ground to the highest point of the shoulders.
Keep in mind that Toy Poodles measure from 10 inches and under as adults. Miniature Poodles measure between 10 - 15 inches at maturity, and Standard Poodles measure over 15 inches at adulthood.
Weight: Most puppies are born about the same size (give or take an ounce). They may even grow and gain weight at the same rate as each other. But as they begin to wean from nursing their mom (usually between 6 - 8 weeks) they are not getting the "fat" from the milk. Then the size difference begins to show.
To estimate the maturity weight of a Toy Poodle you must double the weight and add 1/2 pound at 8 weeks old. At twelve weeks double the weight.
Personally, I have to laugh-out-loud when I see web sites that say that they know that they have a teacup puppy at 2 or 3 weeks old! Parental genetics are a good indicator, but not an absolute. Buyers beware - Poodles are the only breed of dog that has been bred down to create three specific sizes of dog recognized by the American Kennel Club. Because of the "down-size breeding" they all carry recessive genes. Therefore, some larger Toy Poodles can throw Teacup size pups, and some Tiny Toy and Teacup Poodles can throw large size pups.
I am learning as I go, but thought I'd share a little knowledge with those interested and wanting to be "in the know"! And hopefully, this will help you when you are shopping around for a Poodle puppy.