Some reactions have triggers which are tangible events in the fiction of the game. For example, feather fall‘s casting time is:
1 reaction, which you take when you or a creature within 60 feet of you falls.
This is something tangible within the fiction: feather fall’s trigger is something perceived and understood by both the player out of game and the character in game.
On the other hand, some reactions have triggers which are intangible within the fiction of the game: they are perceived only by the player out of game, and do not actually reflect anything about the character’s knowledge in game. For example, the Artificer’s Flash of Genius ability says:
When you or another creature you can see within 30 feet of you makes an ability check or a saving throw, you can use your reaction to add your Intelligence modifier to the roll.
Another example is the Oath of the Watchers Paladin’s Vigilant Rebuke ability:
You’ve learned how to chastise anyone who dares wield beguilements against you and your wards. Whenever you or a creature you can see within 30 feet of you succeeds on an Intelligence, a Wisdom, or a Charisma saving throw, you can use your reaction to deal 2d8 + your Charisma modifier force damage to the creature that forced the saving throw.
Both of these abilities allow the character to take a reaction in response to another character making a saving throw.
But passing, failing, or even attempting a saving throw are not something characters know about. Characters don’t even know what saving throws are. These two questions discuss this:
- In 5e, are saving throw results detected by the opposing party?
- Do characters know if someone else, who they can see, has failed a saving throw?
Suppose the evil Archmage casts scrying on one of our party members, in an attempt to secretly spy on them. Scrying calls for a wisdom saving throw. The party would normally have no idea this was going, no idea someone was making a saving throw, and no idea if that saving throw succeeded or failed.
But what if we have an Artificer or Watchers Paladin around?
Rules as written the artificer or paladin would be able to use their respective reactions in response to the saving throw for the Archmage’s scrying.
Do the artificer and paladin characters know in game that they used their abilities in response to the saving throw against the scrying spell?
Obviously, they don’t know what a saving throw is, and won’t know that it is a scrying spell at work. But do they know that something is afoot and that they used their ability?