Last night, after viewing a forum topic about how to recognize a stunted fish, I realized that my beloved PetSmart, from whom I buy all my fish that are supposed to be babies, may actually be supplying me with a few baby-sized adults.
Fish are stunted through two things: their environment, and their diet. Pet stores overcrowd their goldfish tanks, therefore, the ammonia will build up if there's not proper filtration. In addition, the physical space offered in PetSmart's (or any pet store's) tanks are only enough for maybe one goldfish to live and grow. If the goldfish live in this environment long enough, you may end up buying a stunted fish that will never grow to be the big 8-incher you've been dreaming of.
How can you tell legitimate babies from stunted older goldfish? In my reading, I came across two consistent answers: eyes and fins. If the area above and below your fish's eyes are equal to or less than one eye-height, then your fish is most likely stunted. Bulging eyes are also an indicator of stuntedness. In terms of fns, if a fish's tail fins seem disproportionately big to the fish's body, that is also a clue of stuntedness. Here's pretty helpful photo-manipulation of a normal vs. stunted ranchu. (From this thread in RafflesGold.com Forums)
There are other signs, depending on the breed of goldfish, of stunted growth.
As a bit of a lengthy side note, the easiest (personally) goldfish to identify stunting would be the Oranda. I remember that my Petsmart often carried "young" orandas around 1.5 inches that had impressive wen growth. Seeing this and comparing them to Nano, I would get discouraged. Now I realize that Orandas take about one to two YEARS to develop some wen. This means that these Orandas in PetSmart should have been much bigger in their size for their age. So with Orandas, in addition to the eyes and fin obervations, wen + small size = stunted.
Ryukins and fantails are more tricky though, unfortunately for me. I guess it all comes down to: if the fish just doesn't look right to you, pass.