I'm wondering, how do you know when it's time to change shocks? Is there an objective test the dealer can do to test them, or is it simply a question of how does it ride/handle?
Well drivability is the first good clue.
I don't know if this is the proper way to test them, but I've always just bounced the front or back of the car and seen how many times it continued to bounce after I stopped. It shouldn't continue to bounce more than twice if the shocks are good, at least in my experience.
Also if the shocks are covered in shock oil, obviously they're leaking and are probably going to need to be replaced before long, but some minor leaking may be normal.
Of course there is also the recommended replacement interval of 50,000mi, but I've seen shocks long outlast that before needing replaced.
1996 Mercury Grand Marquis GS ~89,000mi, Willow Green inside and out. It's very green.
1998 Chevrolet Malibu Base V6 187,000mi (sold)