Do players know if a hit from a monster is a critical hit?

Question

There are a few different abilities that PCs can get where once they are hit with an attack they can decide to use to potentially turn the hit into a miss. Examples include the spell Shield or a Cavalier fighter's Warding Maneuver feature.

The way our group currently plays all the DM's rolls are hidden from us and they convey whether attacks hits or misses to us. In the case of a hit the player informally has until the DM rolls damage to decide whether to use a feature that might change the outcome.

While the players are aware of when an attack is a hit, are they aware if the attack is a critical hit? If so they would know that using such features would be a waste of resources.

This would also apply to rare features that can force a reroll after an attack roll is already made such as with the Rune Knight's Runic Shield feature. If a player knows that an attack is a critical hit they would be more inclined to use such features.

Answer 1

Yes, they need to know if an attack is a critical hit

Although there are no rules regarding this as far as I know, as rolling in the open or behind a screen is optional, there are abilities in the game that are reliant on knowing if an attack was a critical hit or not.

As one example, the Grave Domain of the cleric class has the Sentinel at Death's Door feature (XGtE, p. 20):

At 6th level, you gain the ability to impede death’s progress. As a reaction when you or an ally that you can see within 30 feet of you suffers a critical hit, you can turn that attack into a normal hit. Any effects triggered by a critical hit are canceled.

If the player had no way to tell if an attack was a critical hit, this feature would be worthless.

From a DM point of view, I would argue that the players should know when an attack is a crit, for a number of reasons. It builds tension and excitement, and it lets them use the abilities that might save them (the Lucky feat or the Rune Knight fighter's Runic Shield feature (TCoE, p. 45-46)) or not waste the abilities that won't (the shield spell).

Even if rolling in the open, the DM should still state that the hit was critical, as there are monsters and abilities that cause critical hits on values lower than a 20 on the die.

Answer 2

I see no harm in telling them, and if you have a monster that can push a critical hit aside with a special feature you need to know as a DM also. I have always told my players when a hit is critical and have never worried if they use a power of theirs to stop the hit. Often everyone; myself included (DM), cheer and are excited that they got to use a rare power to save themselves.