Lead shield: when equipped by a monster, it prevents you from using probing or a stethoscope on them.

Additional YAFMs that may replace the “You hear your heart beat” message for applying a cursed stethoscope at yourself:

• If hallucinating, “You hear a soothsayer with the weather forecast.” This is followed by either “Gehennom will get a rain of burning pitch later today.” or “The dungeon will be cool, with a stony gray overcast.”
• Otherwise, if undead, “You hear the ocean’s roar.”
• Otherwise, if a Wizard, “You hear yourself mumbling curses.”
• Otherwise, the usual heartbeat message.

When you stethoscope or probe monsters that have names, they are referred to as “[Name] the [species]”, e.g. “Status of Fluffy the large dog:”

Healers can use a stethoscope on a corpse to usually find out what its cause of death was.

Cursed stethoscopes cosmetically stick to your hand when used, but you rip off some skin to get it off when you’re finished using it, which deals 1 damage every time you use one.

Your god may give you useful non-artifact items as sacrifice gifts, such as a stethoscope or bag of holding, so long as you aren’t carrying one of them already.

A slightly more advanced system that wouldn’t require carrying a bunch of items to avoid getting duplicates would be to track the last time you picked up or put down each type of object, and if you have never touched one or it’s been a sufficiently long time, you are eligible to get that item as a gift.

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.

As a Peter Pan reference, produce “You hear a faint ticking noise” occasionally and prior to the normal output when stethoscoping a crocodile. Alternatively, produce this noise very rarely when a crocodile is anywhere on the level or near the player. Possibly only if the crocodile’s monster ID number matches a rare condition, like being a multiple of 100.

Remove the ability of non-blessed stethoscopes to tell you information about undead, so that only a blessed one will work on them. The message for trying to apply it to them is “Foo doesn’t seem to have any vital signs.”

Applying a stethoscope to a monster that has a rusting or corroding passive attack erodes it appropriately.

Stethoscopes should be made of plastic and colored black, not iron and colored cyan.

Stethoscopes don’t work on non-living monsters.

You can use stethoscopes (or possibly probing) on eggs to identify them.

Non-weapon stethoscope skill, which gives less information than it currently does at Unskilled (perhaps just HP), the same amount of information as currently at Basic, more information like pet tameness and apport at Skilled, and a full probe at Expert. Possibly it also takes a full turn to use until Basic or Skilled.

Nurses rarely generate with stethoscopes.

When Schroedinger’s Box is probed or has a stethoscope applied to it, it either reveals the housecat corpse inside or releases the live housecat as normal. Alternatively, stethoscopes give “You hear a characteristic WOM WOM WOM sound” as a reference to Bob the Angry Flower, and probing gives you “The box contains a cat that is neither alive nor dead nor alive nor dead nor… You resolve to call the YSPCA later.”

Using a stethoscope on a monster requires that you succeed in making a to-hit roll against it. Otherwise, it doesn’t work.

Stethoscopes and probing on a pet should show when that pet is satiated.

Applying a stethoscope should tell the target’s gender, and if it’s a pet, the pet’s hunger status.