How to know as a PC how many HD an NPC has?

The title pretty much sums it up. Is there a skill or spell that will allow you to know how many HD or HPs an NPC has also is there a way to know what class levels they have? Thanks.

Fun fact, HD and class levels grow on like extra layers of skin. You can horizontally bisect an adventurer and count the rings.

But in all seriousness, your best bet is to invest ranks into Spellcraft and maybe Knowledge Local. The former will allow you to identify spells, and assuming a 9th level caster is throwing their big guns at you can guess their level within a tiny margin of error. I could have sworn one of the urban themed books has rules for identifying class abilities.

The Phrenology and Prognostication Occult Skill Unlocks can be used to determine class and levels.

Good luck getting someone to let you rub their heads...

Slayer lvl 2 can get Blood Reader, which tells you how many HP remaining a person has.

Also, the Spell Deathwatch (most casters have access to this) or you can get an item, Deathwatch Eyes, 2,000gp.

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Eyes, Deathwatch

Aura faint necromancy; CL 1st; Slot eye; Price 2,000 gp; Weight —

DESCRIPTION

These blood red crystal lenses fit snugly over the wearer’s eyes.

The wearer gains the constant effects of the deathwatch spell.

CONSTRUCTION REQUIREMENTS

Craft Wondrous Item, deathwatch; Cost 1,000 gp.

Trap the Soul says you can research it. I'll just go with appropriate knowledge check.

Actually, Deathwatch does NOT allow you to know how many hit points anyone has. It simply gives you an indication of the range of hit points, very imprecisely.

You instantly know whether each creature within the area is dead, fragile (alive and wounded, with 3 or fewer hit points left), fighting off death (alive with 4 or more hit points), healthy, undead, or neither alive nor dead (such as a construct).

The least practical option:
1. Ensure your target is not immune to energy drain
2. recruit the services of a creature that applies one negative level per hit
3. Debuff the target's saves so there is less error of them saving against energy drain
4. have your draining creature remove one level at a time and count how many it takes for them to die. Be sure to heal hp damage between hits
5 (optional). Resurrect target.

Actually, Deathwatch does NOT allow you to know how many hit points anyone has. It simply gives you an indication of the range of hit points, very imprecisely.

Quote:

You instantly know whether each creature within the area is dead, fragile (alive and wounded, with 3 or fewer hit points left), fighting off death (alive with 4 or more hit points), healthy, undead, or neither alive nor dead (such as a construct).

Yeah, I don’t think there’s anyway to know exact HD except for a successful knowledge check of the monster. Bloodreader and Deathwatch are about as close as you can get without a knowledge check.

For spellcasters, you can probably research what spells they are able to cast and from that guess their level/hd within a level or so of accuracy. For non-spellcasters you could probably research their exploits to get an idea of what kind of monsters they have been known to defeat and kind of guess based on monster CR. But HD/Level are an abstract thing that you really shouldn't know IC. Unless someone has a reputation for being the world's greatest swordsman, you really wouldn't have any idea about how tough they are in a fight until you fight them.

3. Debuff the target's saves so there is less error of them saving against energy drain

Except for the spell, there IS no save vs Energy Drain. The only save you get is after 24 hours, when you try to avoid it becoming permanent. Take a look at the rules.

There are other, similar empyrical methods for guessing at Hit Dice with spells or abilities that have different effects on different Hit Dice, or that only affect a certain number of Hit Dice, like Color Spray, or Sleep, or even Detect Evil/Chaos/Law/Good. Realistically, though, Hit Dice are a game term, not an in-universe term.