But who checks the checker?

Came across this in the Tools For Self Reliance workshop.

It turns out that the label does not refer to the shelf. The metal rule that is usually attached to the wall had been removed for redecoration. I was assured that the rule is carefully aligned using a spirit level. But which spirit level?

This reminded me of a lifelong problem of mine. When they describe something as "the world's most accurate clock", how do they measure it? Surely you would need an even more accurate clock to measure its accuracy . How do you measure the latter's accuracy. And so on