There should be a way to tell what the acquired resistances of a monster are. This could be by zapping a wand of probing at it, by breaking a potion of enlightenment on it, feedback gained from a stethoscope, or (less likely) a new spell exclusively for pet diagnostics.

Note that it may not be considered “fair” for any of these methods to work on pets, but not work to tell the player’s resistances when they do it to themselves.

A cursed potion of enlightenment grants you temporary darkvision.

Monks start with potions or a wand of enlightenment.