Does anyone know how the factory checked the alignment on vehicles rolling off the assembly line? In particular, caster.

On some vehicles, for example the Pontiac Solstice and newest Corvette, there are two machined holes in the hubs to measure caster. The dealer tool for measuring rear suspension caster is a digital angle sensor with two pins that fit into the holes. The angle is read from a remote handheld controller.

Looking at the line drawings in JEPC, the only likely looking candidate is the ABS sensor mounting bore. The bore axis is in the same plane as the plane of interest so any rotation of the bore from the expected angle is equivalent to a change in caster. The face of the bore is also a machined surface perpendicular to the bore so it is also usable as a reference surface.