The lead image of this article shows the chassis (frame and complete drivetrain) being married to the body. From there, the fluids are added and the cars enter the final inspection where workers check the panel gaps and look for paint flaws. See the mirror in the upper left of the picture? It's positioned so the inspectors can verify that the car is wearing the same wheels on both sides. The sign next to the mirror shows the possible wheel options. It sounds elementary, but if you're looking at hundreds of cars per day, it would be easy to forget what wheels are on which side you just looked at.