Above are just a couple of factors from a long list. I wont even go into the details of what our local workshops do (dealerships included). Point is that no matter how expensive of a job you get done, it will always be detected by a watchful eye. Once again, you do not need any expert.
I beg to differ, a good painter can factor in age and everything of the paint and color match according to that. Only foolish and inexperienced painters just go by the color code. That is only the first step of color matching. For instance take the work of Burnees here in Karachi. Experienced "dealers" who buy and sell cars for a living are usually unable to tell where the repaints are, they are that good. Most experienced painters just ensure they only repaint the damaged bits, fade into the surrounding and then get the clear coat over the entire panel and bake it properly. On the other hand, idiots repaint the entire panel and pray that it matches.
The only surefire method of catching repaints is with a device which checks the paint thickness.
First of all, the original paint, which is done in the factory paint booth is baked at a temperature of over 100C while in all workshops, paint is baked at a much lesser temperature of 35-50C because of all the plastics and electricals in the car. Secondly if the original paint is old (i.e. the car is old) and if a panel is repainted, then its texture, shade and feel would be slightly different and no amount of compound polish can fix it.
If you take care of a car's paint properly it will not fade with time, that basically means waxing and paint sealing. I did all this to the last Civic I sold (it was 8 years old at the point of sale) and the factory paint after all these years was still as good as the day it came out because of regular waxing.
If you wax your car properly every week, it will stay fine. Given our harsh environment (extremely dusty) there are ways of waxing but thats' a different topic altogether. People here don't know how to wax at all. They think applying a thick coat of turtle wax does the job. lol.