A good question; and one not easily answered - I doubt that there's an easy "one size fits all" solution.

Did you see this earlier post?

For reference:-

1) Welch-Allyn
2) Voltcraft

To be clear, when you say "validate" do you have in mind "check" (as in, good enough for acceptance testing, and (or) regular PM), or "calibrate" (as in, following repair)?

If you don't inspect ... don't expect.