When does a player have to state they are making a passive check?

Question

So going off of these parts of passive checks

"Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster." (PHB, 175)

"Passive Perception. When you hide, there's a chance someone will notice you even if they aren't searching." (PHB 177)

The section on passive perception versus stealth shows that the person just notices without even trying, which is different from "oh I'm gonna keep an eye out." You just innately and immediately see them. This infers that this innate form of passive check can sometimes activate without the player saying anything.

So the issue is, when does a player have to state they're attempting a skill before the passive check kicks in?

Like, I could see if a passive insight check beats a deception roll, the person realizes they're being told a lie, without first having to say they're trying to use insight on the person. While you lose the part of the game of player's having to carefully consider each part of the conversation to see which parts they want to call insight on, some player's could just state "I insight them every time they tell me something.

And then if it's like an intelligence check, should a player first have to state that they want to make the check? Is it they walk into a room with an obscure holy symbol on the wall, their passive religion checks beats the DC to recognize it and "as you walk in, you see a symbol on the wall and immediately recognize it as the holy symbol of 'so-and-so'," or do you wait until they state that they want to examine the symbol?

Answer 1

Players Do Not Initiate Passive Checks

The operative word is passive. If a player is actively searching/examining/studying/watching, it's active, not passive. If a PC is actively using a skill, they roll for it instead of the DM using a secret passive check.

The only exception to that rule is the one you quoted: the DM can use a passive check to find the average result of doing the same thing over and over again, regardless of whether it's active or not.

In general, the point of the existence of the passive check is so that the DM can determine something when the players aren't expecting to notice anything. In those cases asking them to roll would give away that there is something, so instead D&D 5e provides DMs the passive check to avoid giving away the secret. When to use passive checks is much easier to grasp when you look at them from the perspective of the problem they were designed to solve.

Also as a consequence, PCs are rarely in situations where it makes sense for players to ask for passive checks — after all you can't know to check what you don't know is there to check! This puts the responsibility for ensuring passive checks are made when they should be on the DM's shoulders. As a useful aide for this, it helps for the DM to keep a note listing each PC's name and their most important passive checks. Just having the note can be a small spur to remind the DM that passive checks should be kept in mind during interactions with hidden things, NPCs, motives, and etc.

To answer the titular question, this means that a player never states that they're making a passive check. It's something the DM does as the situation requires.

Answer 2

Consider different approach

Should a player first have to state that they want to make the check?

This very question emerges only because your players have to announce checks. It happens when players use the CRPG-style "I use ___ skill" pattern:

— "Stop right now! Who are you?" the guard says.
— I use Deception skill. (rolls a die)

Aside from its general awkwardness, this approach has a number of issues. How the player is trying to convince the guard? What are possible consequences, what happens when the bluff will be revealed? What if the guard actually knows who the characters are, and will recognize any obvious lie? And what lie does the PC say, exactly?

In 5e, the rules were reworked in favor of storytelling. That includes major overhaul of the skill system - the number of skills was lowered, and their descriptions became very general.

Players aren't supposed to ask for checks

In 5e, a standard interaction between GM and players is:

1. DM describes the situation
2. Players describe what they do
3. DM asks players to make ability checks (if any)
4. DM resolves the situation, describing the outcome

Instead of players saying "I want to roll the __ check", it is the DM who asks the player to make a check:

DM: "Stop right now! Who are you?" the guard says.
P: "Let us pass! We have a message from your king!"
DM: Make a deception check.
P: rolls 2
DM: "Our king is in his chambers, you impostors! Lay down your weapons and surrender!"

Every time the DM wants a player to make a check, he might use a passive check instead. It can be useful in some cases:

• DM doesn't want to "warn" the player asking him for a roll
• There isn't much sense in rolling the die, since the task the PC is performing is trivial or repetitive:

P: I search for traps in the room.
DM: Make an Investigation check
P: (rolls a die)
DM: You've found nothing.
P: There must be something here! I search again.
DM: (uses passive check instead)

However, there is nothing wrong if the DM don't use passive checks at all, but every time asks for a roll instead (or rolls the dice secretly behind the screen).

Answer 3

Passive checks are exactly that. They need no player input. Passing a passive check to notice something implies you were keeping your eye out. An active check would be if you were searching for something.

Generally, players don't announce things like that.

You tell them if they notice. You roll their passive check. If players are announcing passive checks, somethings wrong.