For me, I would just grab the forks where the legs meet the stanchions and see if there is any slop. What really matters is the bushing to stanchion interface. The bushing will also wear, but obviously they are a somewhat cheap thing to replace, compared to new stanchions.
to accurately measure the stanchions you would want a micrometer as the calipers would likely not be accurate enough. If you were to find noticeable wear via a set of calipers, you would likely be able to easily feel the wear via just using your hands.
hope that made sense.
