What check to use for finding out current hit points?

Question

There are Words of Power in Pathfinder, like the Power Word Kill and other similar Words for "Blind" and "Stun", which affect creatures according to their current hit points. E.g. any creature having more than 100 hit points is unaffected, any other creature instantly dies.

I'm a GM, and I am sure that my players will ask me how they can find out how many HP a target has before they cast the Word of Power.

What would be a fitting check for finding out, and what would be the DC? Heal? Perception? some Knowledge skill?

Answer 1

A creature typically can't determine another creature's remaining hit points

However, a creature can come close using spells like deathwatch and detect animals and plants, and optional rules like Wound Thresholds will allow more accurate guesses as to a creature's hp, but, overall, a creature's current hp is usually information possessed only by the GM.

Since Pathfinder monsters typically have a predictable number of hp based on CR, a successful Knowledge skill check may give a canny player a means to guess at a particular monster's hit points and, further, allow the PC to glean as "a piece of useful information" how many Hit Dice the creature possesses, if the GM's comfortable revealing that as one of the creature's "special powers or vulnerabilities."

The advantage of the power word spells is that they are ranged spells with no saving throws, a virtual saving throw being provided by the creature's remaining hp. And, while I know the power word spells' disadvantages are many, a caster that knows one or more of such spells and also knows a monster's remaining hp eliminates any guesswork in a way that's superior to the typical caster knowing a monster's current actual Fort, Ref, and Will saving throw bonuses! If a hp determination technique's available, a power word spell's caster no longer need guess and can word foes with impunity. Hence, unless a special ability or spell grants such an ability, I'd advise against a house rule permitting a creature to determine another creature's hp.

Answer 2

The usual check for medical analisys is Heal.
For the DC I'd tie it to the creature type, knowing the health status of an human is fairly easy for the trained eye, knowing the health status of an ooze or a ghost not so much.
I'd also consider situational modifiers, wizards like to stay as far away from monsters as possible, assessing the condition of a medium-sized human 100 feet away should give a rough result at best.

Bare in mind, you have no obbligation to tell your players the exact amount of HP a creature has. Hit Points are an abstraction, they don't exist in-character.

In fact, I'd never answer a check like this with the crude HP number.
"It's barely/moderately/seriously wounded." is the answer I'd give most of the times.
"It's at 90/80/70% of its maximum." for very very good check results.