This photo (which can be greatly enlarged by clicking on it) shows the diagnostics that will allow the online coin buyer to evaluate PCGS slab photos for himself. The slab on the left is a Chinese-made fake PCGS slab, which was first published by Random House in Scott A. Travers' Coin Collector's Survival Manual, 6th Ed., (without these markings.) The PCGS slab on the right is genuine. The markings were made by a Chinese counterfeiter.
At the time I am publishing this, PCGS has never made these diagnostics public, for fear that they would educate the Chinese counterfeiters in what they were doing wrong. However, making counterfeits and reproductions of things is a time-honored Chinese art form dating back into Chinese antiquity, and the Chinese counterfeiter who claims he made the fake PCGS slab in this photo says he knows what he is doing wrong, but is still working to fix his "challenge in the making perfection of" (i.e. engineering challenges.)
By examining his markings and comparing the differences between the two PCGS slabs, you should be able to discern the visual differences between the two.