I doubt there will ever be an official metric, so it's your word against theirs.
I, for one, find the narrative that "they're just making it up so they can extort their employer for more money" to be less believable than than simply considering that it's the truth.
Furthermore, it was pointed out earlier in this thread that employees discuss engaging in this behavior _amongst themselves_. If it is, in fact, completely untrue, you would have to add "widespread conspiracy" to your acceptable narrative. (My reasoning being that there's far less incentive to lie to a co-worker than to a boss; only the boss can affect change. The notion that 260k low level employees have banded together to promote a lie, including amongst themselves, is so unlikely I'd call it laughable.)