Ask why it runs: user did something means handler, screen must stay in sync means effect.

An event handler runs in direct response to something the user did, such as a click or a submit. An effect runs because the component is on screen and needs to stay synchronised with something outside React, such as a timer, a subscription or data for the current id. Look at the cause of the code, not at how technical it feels.

Read it once, then a question tests whether the trick stuck.