Find on which WordPress hook something has happened

which-hook.php

<?php
/**
 * In this example, we will test has WooCommerce populated the global $product variable.
 *
 * Basically, we add an action to the WordPress `all` hook, then `error_log` when the criteria is true, and unhook.
 */


global $has_product_global_been_set;
$has_product_global_been_set = function() {
	global $product;
	if( !empty( $product ) ) {
		$was_set = current_action();
		error_log( "global \$product first set on action {$was_set}" );
		global $has_product_global_been_set;
		remove_action( 'all', $has_product_global_been_set, 999 );
	}
};
add_action( 'all', $has_product_global_been_set, 999 );