Based on what they want their PCs to do.
In the example of play in the AW rulebook, Marie's player wants (as Marie) to visit grief on Isle. So goes to look for her. That does not require a dice roll but rather simply prompts the GM to say something. The GM, following the principles, chooses to tell Marie's player that she finds Isle (and two other people) sitting on the roof of a car eating tinned peaches.
Marie's player declares that she (as Marie) walks past them and uses her mental powers to compel Isle to follow her. This requires a roll, and the result of the roll requires the GM to decide whether Isle does what she wants, or suffers (what D&D would call) psychic damage. The GM opts for the latter, and describes Isle's head lolling onto her boyfriend's shoulder.
Marie's player, like Marie, is disappointed by this outcome. And so goes back home. As this doesn't require a roll, the GM says what happens next, and describes noises at Marie's door after she's been home for a few hours - the noises are Isle's friends coming to get revenge against Marie. And then asks Marie what she does.
Etc.