Can't you simply test by asking what happens if the person stops doing their job? If nothing detrimental happens, then the job was probably bullshit.

some consequences have feed-back loops with long delays.

go remove some guys that do legal compliance stuff and when the random inspection happen years later, you'll find out that maybe it would've cost you less to keep that employee.

That is definitely true. One would need to carefully consider what the person does before making such a judgement.

Planes can keep flying without a pilot for quite awhile, until they run out of fuel or crash into a mountain.

IIRC one of the examples in the book is someone who just stopped doing their job or showing up and nothing bad happened, and the bureaucracy of their organization kept paying them.

Without even specifying the length of time to measure what happens or the nature of job in question?

Bad news: you have been rejected from consideration for any work related to realtime/life critical systems.

Good news: you may have a future in politics, at least in the county I live in.

Ideally the point is to ask this question before you have them stop doing the job.

"What will happen if this person stopped doing their job?"

"Oh the important monthly reports will stop going out."

"Oh, okay, we better not have them stop doing that then."