Awareness of Being Targeted. Unless a spell has a perceptible effect, a creature doesn't know it was targeted by the spell. An effect like lightning is obvious, but a more subtle effect, such as an attempt to read thoughts, goes unnoticed unless a spell's description says otherwise.
Invalid Targets. If you cast a spell on someone or something that can't be affected by it, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended.
If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn't attempt one (giving no hint that the creature is an invalid target). Otherwise, you perceive that the spell did nothing to the target.
So, the PHB says this regarding spell targets. Which probably implies you don't even necessarily know if a target was charmed by e.g. your Charm Person or not (I mean, the target wouldn't be aware, and the spell doesn't state anywhere that you know)? But the Fey Wanderer's Beguiling Twist (level 7 feature) presumes that you do know? How does that work? Even if you just tell them whether their target was charmed or not ‒ they wouldn't know whether a saving throw was attempted (if for example the presumed humanoid they cast Charm Person on was actually a fiend in disguise), but they have to know to know whether they can use that reaction or not. Or what if an enemy tries to unsuccessfully charm (or frighten) an ally, they similarly wouldn't necessarily know about the saving throw or that the condition was even attempted to be inflicted? Do reactions in general require just player/DM or also PC/NPC knowledge? (if the former, the DM could tell the player so they could activate their PC's reaction, while pointing out their PC doesn't know of whatever happened, just that they somehow used an ability to try to charm/frighten someone of their choice)
Similarly, you're not usually aware if you're being targeted by some source of the charmed condition, consequently the PC doesn't know whether they're charmed or not ‒ however, the player does know ‒ so is the Monk's Self-Restoration (level 10) feature something that requires only player knowledge, or does the PC need to be aware as well?
[...] So, the PHB says this regarding spell targets. Which probably implies you don't even necessarily know if a target was charmed by e.g. your Charm Person or not (I mean, the target wouldn't be aware, and the spell doesn't state anywhere that you know)? But the Fey Wanderer's Beguiling Twist (level 7 feature) presumes that you do know? How does that work? Even if you just tell them whether their target was charmed or not ‒ they wouldn't know whether a saving throw was attempted (if for example the presumed humanoid they cast Charm Person on was actually a fiend in disguise), but they have to know to know whether they can use that reaction or not. Or what if an enemy tries to unsuccessfully charm (or frighten) an ally, they similarly wouldn't necessarily know about the saving throw or that the condition was even attempted to be inflicted? Do reactions in general require just player/DM or also PC/NPC knowledge? (if the former, the DM could tell the player so they could activate their PC's reaction, while pointing out their PC doesn't know of whatever happened, just that they somehow used an ability to try to charm/frighten someone of their choice)
In my opinion, some game mechanics and PC abilities require sometimes meta knowledge to work, for example the one you mentioned: Beguiling Twist.
After all, the player needs to roll with Advantage, but if the creature succeeds on the saving throw, it won't know it was targeted by Charm Person. Roleplaying the situation can be interesting in both cases.
Similarly, sometimes meta knowledge is required for the second part of the Beguiling Twist feature:
In addition, whenever you or a creature you can see within 120 feet of you succeeds on a saving throw to avoid or end the Charmed or Frightened condition, you can take a Reaction to force a different creature you can see within 120 feet of yourself to make a Wisdom save against your spell save DC. On a failed save, the target is Charmed or Frightened (your choice) for 1 minute. The target repeats the save at the end of each of its turns, ending the effect on itself on a success.
Similarly, you're not usually aware if you're being targeted by some source of the charmed condition, consequently the PC doesn't know whether they're charmed or not ‒ however, the player does know ‒ so is the Monk's Self-Restoration (level 10) feature something that requires only player knowledge, or does the PC need to be aware as well?
The awareness should depend on the origin of the Charmed, Frightened, or Poisoned condition. But in any case, the player will know, and then the character can use the feature.
So, the PHB says this regarding spell targets. Which probably implies you don't even necessarily know if a target was charmed by e.g. your Charm Person or not (I mean, the target wouldn't be aware, and the spell doesn't state anywhere that you know)? But the Fey Wanderer's Beguiling Twist (level 7 feature) presumes that you do know? How does that work? Even if you just tell them whether their target was charmed or not ‒ they wouldn't know whether a saving throw was attempted (if for example the presumed humanoid they cast Charm Person on was actually a fiend in disguise), but they have to know to know whether they can use that reaction or not. Or what if an enemy tries to unsuccessfully charm (or frighten) an ally, they similarly wouldn't necessarily know about the saving throw or that the condition was even attempted to be inflicted? Do reactions in general require just player/DM or also PC/NPC knowledge? (if the former, the DM could tell the player so they could activate their PC's reaction, while pointing out their PC doesn't know of whatever happened, just that they somehow used an ability to try to charm/frighten someone of their choice)
Similarly, you're not usually aware if you're being targeted by some source of the charmed condition, consequently the PC doesn't know whether they're charmed or not ‒ however, the player does know ‒ so is the Monk's Self-Restoration (level 10) feature something that requires only player knowledge, or does the PC need to be aware as well?
In my opinion, some game mechanics and PC abilities require sometimes meta knowledge to work, for example the one you mentioned: Beguiling Twist.
After all, the player needs to roll with Advantage, but if the creature succeeds on the saving throw, it won't know it was targeted by Charm Person. Roleplaying the situation can be interesting in both cases.
Similarly, sometimes meta knowledge is required for the second part of the Beguiling Twist feature:
The awareness should depend on the origin of the Charmed, Frightened, or Poisoned condition. But in any case, the player will know, and then the character can use the feature.