a rare moment of usefulness from Wikipedia:

Quis custodiet ipsos custodes? is a Latin phrase found in the work of the Roman poet Juvenal from his Satires (Satire VI, lines 347–348). It is literally translated as “Who will guard the guards themselves?,” though it is also known by variant translations.

The original context deals with the problem of ensuring marital fidelity, though it is now commonly used more generally to refer to the problem of controlling the actions of persons in positions of power, an issue discussed by Plato in the Republic.

I prefer the line “who watches the watchmen” although I more often use the phrase to discuss the common use in terms of possible and impossible mechanisms of accountability for persons in power, rather than simple assumptions of corruptibility.