How do you calibrate a magical item?
If it does something like weigh stuff, you can do it like any other mundane item, measure it against the standard. however, why would you bother?
It's the belt that's supposed to fasten only about a faithful wife's waist, a cup that only the honest can drink from, a sword that is for the finest knight, that are tricky. If it were so easy to find a test case by mundane means, you wouldn't have devised a magic test. And even if you could have two, a success and a failure, perhaps that was coincidence. Tales seem to assume by the definition a wizard always makes what he intends.
Here's a plot bunny free to a good home: they use a magical stone, carved with a mouth that pronounces "True" or "False," except one day, some mischievous soul asks if it always tells the truth, and it answers "False." Uproar ensues.