We routinely will measure paint thickness and although there is some variability we typically see consistency on similar panels. On a GT, I wouldn't be particularly alarmed but if there are other "tells", then it becomes more suspect. For example, on one car we inspected a few years ago, Ryan noticed that the fasteners used in the driver side wheel well - while nice fasteners, they were not OEM. That front left wheel was also the only wheel on the car that did not have OEM wheel weights. Finally, the paint thickness on that fender was substantially different (thicker) than the other front fender. While we could never be 100% certain, we would say that the probability of paint/body work on that front corner was higher than what we would like to see.