Pete, growing a cultivar to harvest can help but, as I already said, it is not going to provide 100% confidence. Again, fruits appear differently in different climates. Nobody has mentioned leaves yet. I do compare leaves and use that as a guide. One tree I bought from another member here did not match up and I pointed that out to him and he apologized and made a refund (which I said beforehand was not being requested). Once that tree fruits for me I still won't be selling it as a named variety as I already know it's wrong.
Hobbyists can come up with many different ideas of what they would like to see put into practice but that does mean it will be put into commercial practice. Most nurseries in the USA do not grow fruit/nuts on their own mother trees. For instance, when UC Riverside bred and released the citrus variety 'Tango', it was released through CCPP with a limit of 12 buds per nursery. Within a year, there were hundreds of thousands of Tango trees being sold. Some people here at F4F have bought trees from Burnt Ridge Nursery. This is a small nursery and I visited the place in 2000. I am pretty certain that owner Michael Dolan doesn't produce any fig fruits yet he sells fig trees. At the same time, he does produce different chestnuts and also sell chestnut trees. Many of his chestnut trees that he sells are mislabeled. I know this from first hand experience as well as from hearing from several others who have experienced the same problems. So how well does the fact that his mother trees produce nuts help? Duarte Nursery in California produces something more than 10 million trees and vines each year and most of these are varieties that they don't fruit. Many are produced in tissue culture and then grown in greeenhouses. This is a commercially accepted practice. If I'm not mistaken, Jon has also sold cuttings from varieties that have not yet fruited for him. Doing so helps get the diversity spread out more quickly.
The cuttings I've sold have made it clear if they have fruited for me or not. I make a careful judgment on the source and take great care in labeling of my new plants (rarely, I find a plant that has a lost label and I through the plant away). I believe my actions are prudent.
By the way, Socorro Black to produce fruits for me, but they were too late in the year to fully mature so I did not even try eating them or take photos.