True test for underfocused camera and what not?

Ok, I hear that some people send their camera's for tuning, adjusted for being underexposed and so forth...

Are there true tests for actually seeing whether if a camera is shooting underexposed, or if the light meter is not metering correctly, etc. etc.

You can always shoot a white piece of paper without exposure comp, then put it in Photoshop and it should measure around 12-18% gray.