Checking parallelism of mike anvils

[mklotz]

Use a ball-bearing ball. They have tightly controlled sphericity (millionths) and, with the point contact, can be "walked around" the anvils to check them for the same reading at various places.

[Toolmaker51]

On a tack from Marv's post, bearing balls are the very best (short of calibration devices) check of vernier or digital caliper faces.
The nature of the beam versus slide portion, puts wear on roughly 0.25% to 0.75% the full range. You can often feel slight difference in friction and play operating one, a sphere will display it, due to exaggerated distance from slide to measuring tip.
A small reamer blank held lengthwise with lightest possible contact does a good comparison of entire surfaces. More telling than closing them to examine, while held up to a bright source of light. It's wide contact versus line contact, same as knife edge squares and straight edges.