Does a character / player "know" if his ability worked?

[Helel13]

Basically title. This is both an in character thing and to an extent a mechanical thing

If a Paladin Smites Evil, does he know if the Smite works?

If a spell effect is not immediately obvious, for example an Evil Eye hex debuff, should the player know if an enemy is immune to it and it fails? Do I roll the Will save anyway if an enemy is of a type that isn't affected by mind-affecting spells/abilities to keep up the "illusion" of the player/character not necessarily knowing an enemy's resistances/immunities unless they roll a Knowledge check?

[Darksol the Painbringer]

That's really up to the GM.

There are some precedents regarding other rules, though they are conflicting. For example, if an enemy succeeded a Will Save from a spell (like Charm Person), they know that they were the target of a spell, even though the spell has no obvious effects. Although that alone doesn't tell them who cast the spell, and what the spell is (that's what a Spellcraft check is for), all they know is that whatever happened, it didn't work.

Similarly, if a PC makes an attack against a creature with DR, the GM tells the players that their attacks aren't particularly effective (though the GM may leave out how little their attacks are doing), or that the attacks don't do anything at all.

With these precedents, it's a coin toss, and should be played on a case-by-case basis.

With the Paladin Smite, it should be obvious to the PC whether its effects are active or not. Your attacks will be doing increased damage, have increased accuracy, and be able to bypass any DR the enemy possesses, all of which is grounds for telling the PC the effects of their ability (if any). You'll also have obviously increased AC as well, and should be relevant for the PC to know for when an enemy chooses to attack them, so that when you ask what their AC is, they add it up properly.

Evil Eye is similar to the DR argument, the GM should tell the PCs when a creature has a resistance/reduction or immunity to an effect when that effect is used on that creature. Obviously, effects which fail to work normally should not be granted the extra information. However, effects like Evil Eye have a partial effect, even on a failed saving throw, so in this case, you should tell them regardless (though on any other Negates saving throw, the information can be withheld until a failed saving throw occurs, because they aren't sure whether they made the saving throw or are just immune to the effect).

[Pizza Lord]

Darksol's advice is pretty good. In the case of knowing your ability failed on a creature however I would not tell them why. They don't know. You should roll a couple dice anyway.

For example, if a PC tried to charm person on a Hag (monstrous humanoid) disguised as a human the spell will fail. The player only knows the spell has failed. They don't get to know why. It could be because the target isn't humanoid, it could be because of spell resistance, it could be because the target passed the will save, it could be that the target is standing in a globe of invulnerability, or even have protection from good/evil which prevents new charms and compulsions.

If you tell the player that it fails because the target isn't humanoid you are giving the PC knowledge they shouldn't have and that isn't fair to the player. You've taken the mystery and the surprise and the value of suddenly finding out a secret away from them. They may have tried to cast a second charm or taken another action and you will have taken the freedom of choosing away (A mature and proper role-player will still keep player/character knowledge separate, but now they'll just feel like a schmuck doing the action, especially if the rest of the party is in trouble).