It's true that they must meet the standard to be a true heritage breed bird, I'm not an expert, but my understanding is that there is really no 'control' of it, to get a true heritage bird you need to deal with a reputable breeder and do your homework. A reputable breeder really cares about the standard for the breed they are raising. I suppose if you are showing birds, or breeding for sale, it would be much more comprehensive as far as proof, I don't show so I'm not really knowledgeable about that. I know that most hatchery chicks sold as Rhode Island Reds for example, don't meet the standard for the breed (they are not nearly as deeply colored), and are probably from very impure lines. I have bought 'heritage breed' birds locally that have done very well, whether they actually met the strict standard or not, I don't know. I just wanted healthy birds, and seem to have gotten them, and they meet my needs and wants. I also have hatchery birds knowing that they may have shorter lives, and mixes of the two. I have a New Hampshire Red hen that is at least 8 years old, and my best broody. Breed matters, but so does individual genetics.
If you start looking around your area, however far you are comfortable going, you may find some of them, some breeds can be more common in some areas.