Can your character tell exactly how much damage they have dealt?

Question

Say you make a melee attack, would your character reasonably be able to tell what a good hit is versus a mediocre one? One that hits for maximum damage, or even a critical hit, versus a minimum value damage roll.

I know as a player your damage is known to you and can be altered after the fact with several re-roll features such as Piercer or Great Weapon Fighting. Is there anything in any sources that would indicate a character understand the damage it is dealing?

There’s an edge case that came up when using the UA Monster Hunter fighter subclass when a player expends superiority dice:

If the target of your attack is an aberration, a fey, a fiend, or an undead, you deal maximum damage with both dice, instead of rolling them.

I am trying to figure out if this can reasonably be used to figure out if a disguised creature is secretly a fey/fiend/undead/etc. via attacking them and gauging the damage dealt.

Answer 1

First, yes, your characters should be aware of the effect they're having on a target. Mechanics are abstractions of what's happening in the game-world, but they still represent things that are actually occurring. The characters may not know what a hit point is, but they know when they deal a mighty blow (even if the monster shrugs it off because they're just that tough).

It should be pretty immediately apparent when you're hitting resistance or vulnerability, for example, and critical hits are clearly something the character knows about, since making a critical hit can trigger other effects (such as Great Weapon Master's additional attack). It's entirely reasonable that the character knows when they made a solid hit or when they just barely made contact with the target, since that should be a visible, palpable effect.

But that brings up a new question. Do certain class abilities work when the target has a specific type, but the character is unaware of that fact? That's an interesting question that has nothing to do with whether characters know how much damage they're doing.

Some 'favored enemy'-type abilities probably don't care whether the character knows or not -- for example, a paladin's smite deals increased damage to fiends and undead, and that should hold true even if the paladin is unaware of the target's true nature. The extra damage is happening because the magic of the smite is inimical to the target's nature. The paladin should probably be aware that their smite worked better than expected on the target and be able to extrapolate important information from that.

But should a 20th level Ranger be able to use Foe Slayer if they don't KNOW they're fighting a favored enemy? The ability seems to imply that you're able to hit harder or more accurately because you know how that kind of creature acts, which makes no sense if you think they're a different type of creature. (Trying to use it and having it fail might be a giveaway that you're dealing with a shapechanger of some sort, though.)

The Monster Hunter's ability seems like the latter type -- it's based on experience dealing with a particular monster type, which should only apply if you know you're dealing with that monster type.

But I guess it's up to the DM to make that call, and they could decide that your ability to hit fey creatures hard still applies, even if you aren't aware they're fey.

Answer 2

The game discourages using mechanics this way, but your DM decides how to handle it

The PHB says about the observable effects of damage on p. 197:

Describing the Effects of Damage

Dungeon Masters describe hit point loss in different ways. When your current hit point total is half or more of your hit point maximum, you typically show no signs of injury. When you drop below half your hit point maximum, you show signs of wear, such as cuts and bruises. An attack that reduces you to 0 hit points strikes you directly, leaving a bleeding injury or other trauma, or it simply knocks you unconscious.

The effect that damage dealt has on a creature is not only a function of the amount of damage. It is a function of the amount of damage and the number of hit points that creature has. When you deal 10 damage to a commoner, you slay that commoner on the spot. Deal the same 10 damage to a hill giant, and you will not even see a scratch.

There is the general question why the characters in the game world would not be able to infer the game mechanics over time, when they can observe their effects again and again. And there is no explicit statement in the rules that they cannot do that.

From the way the rules are written overall, addressing mostly the player when it comes to game mechanics, it however is clear that the rules are a tool for us, the players to be able to resolve the outcome of actions. They must by necessity simplify to be playable. But the idea is that they represent a world such as ours — with mundane swords, wounds and diseases, that work as they would in the real world.

The player characters generally would not know about things like hit points, they would know about being wounded or tired. The quote above shows that the severity of damage dealt is difficult to quantify for the characters — until they have dealt half a creatures hp, they cannot even see any wounds.

The DMG advises to play without thinking about the game in terms of mechanics on p. 235:

Discourage metagame thinking by giving players a gentle reminder: "What do your characters think?"

How you play this is however entirely up to you. Some tables and DMs have no problem for stating the exact numbers and sharing if it is maximal and the character acting in some way on this knowledge. You can even couch this in language like "you feel how your strike digs into the monster as if guided by a supernatural force", to provide in-game explanation to the characters.

Other tables and DMs keep all of this secret, let you roll but behind the screen record maximum damage.

I personally think that letting the character experience their special ability is at work, while keeping the monster hp secret apart from if the monster is physically wounded or maybe very close to death is a good balance, but it is really up to the individual DM and table and how they want to play.