Uhhh... what spell are you using?

For those who don't get it

Power Word Kill

You utter a word of power that can compel one creature you can see within range to die instantly. If the creature you choose has 100 hit points or fewer, it dies. Otherwise, the spell has no effect.

~~Easy to make misstake, but~~ it was Power Word: Pain, ~~not Kill~~.

You speak a word of power that causes waves of intense pain to assail one creature you can see within range. If the target has 100 hit points or fewer, it is subject to crippling pain. Otherwise, the spell has no effect on it. A target is also unaffected if it is immune to being charmed.
- Crucio!
I would not give an NPC a spell like that so I suggest this:

Power Word Incapacitating Tickles

You utter a word of power that can compel one creature you can hear within range to fall over squirming and laughing, begging you to stop the imaginary tickles until you choose to end it's effect. If the target has less than 100 hit points it takes effect, otherwise the target enjoys the sensation that feels like tummy rubs and is charmed for one round.
- When they put you against someone using it in Baldur's gate 3 the make it a three turn ritual, so you just need to break the caster's focus and you get a few goes at it

Sorry, why did you need to math your hit points? Were you not keeping track already?

One of my friends started doing a thing where he adds his damage together. When it's as high as his hit point total, then he's down. I am trying it out. So I just needed to math the subtraction. Not a lot of math but shrug.
- Makes sense. One of those "it's easier to add than subtract" things. I probably won't do that, but if it works for you, then that's good.

As a DM you should not be asking your players about their HP totals directly in order to target them with Power Words (or Sleep)

As a DM, I don't think you should be using Power Word Kill at all. "I've set up this awesome encounter that you're all going to love. Except you. You'll have to sit this one out."
- There are ways. You could, for example, set up a bbeg where that's his whole deal. The townsfolk are scared of this guy because he has the supernatural power to just kill you, straight-up. Maybe the questline leading up to their encounter involves the players finding defenses or counters or sabotaging his supply of spell components or whatever, such that, if they DO get power-word-killed, it's because they had ample opportunities to not, and failed to take them.
- Makes for a great BBEG move to use against a favorite DMPC or other beloved party companion, though. You don't use it on a player, you use it on their feelings.
- Can make for some epic moments depending on the party and setting and fight etc. My character in the first stage of the final fight of the campaign got power word killed after getting the BBEGs lair action enabling staff away from him, because taking the staff hurt him enough to put him under 100. Incredible narrative moment, didn't expect the disarm to work and he passed it off with his dying breath. Got revived in a later stage once reinforcements started showing up
- It's definitely one of those things that you shouldn't just throw out there. Gotta plan for it
If it is to see whether it takes effect I think it is fine if it is to know whether or not to target them I completely agree that it is not fun or fair.
- Yeah, of course you need to know to resolve the effect
Quite possibly the enemies have a better idea of the players' hitpoint total than the DM, since they can, like, see how the player characters look, and probably have been tracking the battle much more closely than the DM has.
I agree. The enemy considering using the spell doesn’t know the HP totals. Why should the DM controlling that enemy?
- from what i understood, the dm asked to know if the spell would have effect, not if the creature was going to use it or not
- Because the DM needs to know if the spell succeeds before they can narrate anything.
Whoever downvoted, I'd be interested in hearing why you disagree.
- I also didn't downvote, but maybe you would want to convey the information to your players that creatures here can use that spell without outright killing a PC in the first battle? Like "you guys should really watch your health points in this area" type of thing
- I didn't down-voted. However, I would somehow disagree. Knowing how badly injured are the PC allows to adapt the tactic, in both direction. Yes the warrior is a tank who can take tons of hit (is there really games where PC have 100 HP ? that much ? ) and you have that social character who can't do much in combat, who is usually unconscious at the end of the first-round. So knowing how much HP do the PC have helps managing the right level of power against them and avoid stupid character death.

Do you not keep up with your own HP?

I have a guy who does that for me and yells a lot about positioning.
- I used to ask people to tell me if they need healing, but I found it easier to just keep track of it all myself so that I can plan when to switch roles from buff to heal.
  
  Combat encounters usually have me bringing out a calculator and a sheet of paper where everyone's hp is being tracked.

RIP-Rest in Pieces