Too_Many_Tools writes:

So, how does one decide if a surface plate is "good"?

There is a chapter on this in Connelly's _Machine Tool Reconditiong_.

Ordinary gages and indicators are not precise enough to calibrate a surface
plate, although they could detect gross problems. You're in the realm of
ultra-flat reference gages and solvent-evaporation marking.

Being granite, it probably is still good as new. Minor surface dings are
of no consequence. If you inspect it closely with glaring reflected light,
you should see the original polished surface quality, as opposed to
dullness from misuse or abuse.