Here's what it says here <Using Ability Scores>:the 5e rules state that when the DM has uncertainty, they should roll a die.
The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results.
For every ability check, the DM decides which of the six abilities is relevant to the task at hand and the difficulty of the task, represented by a Difficulty Class.
For every ability check, the DM decides which of the six abilities is relevant to the task at hand and the difficulty of the task, represented by a Difficulty Class.
It clearly states that the GM decides which ability is used to make the check, and what the difficulty is. But it doesn't say that the DM calls for a roll if the DM has uncertainty. It says that a roll is to be called for, by the GM, when an attempted action has a chance of failure and [thus?] the action's outcome is uncertain. (The wording is a bit unclear: I assume that it is meant to imply that the chance of failure must be less than 100% for a roll to be called for. Although the rules don't seem to preclude setting a DC which makes the chance of failure 100% - eg a DC 20 for a check being rolled with a -1 penalty for having an 8 in the relevant ability score and no other modifiers.)
It seems to me to leave it open how the table decides that the outcome is uncertain.
It also refers a monster attempting an action. So what happens if an Orc warlord tries to drive off a PC with a fierce bellow? Who decides if the outcome of that action is uncertain? Is the GM allowed to decide that it is, and therefore make an ability check for the Orc to see if the action succeeds?
This is why I posted upthread that
To me, the 5e D&D rules on what skills are for, and how they relate to the fiction - both fictional positioning, like Am I free to just declare that I cut down my friend without consequences? and fictional consequences, like Am I just free to declare that my PC is unpersuaded by the charlatan, although the charlatan has hoodwinked everyone else they've ever dealt with? - seem pretty unclear.


