Debug WordPress admin menu – on dashboard only

wp-admin-menu-debug.php

<?php
if ( ! function_exists('debug_admin_menus')) {
	function debug_admin_menus()
	{
		global $submenu, $menu, $pagenow;
		if (current_user_can('manage_options')) {
			if ($pagenow === 'index.php') {
				echo '<pre>';
				print_r($menu);
				echo '</pre>';
				echo '<pre>';
				print_r($submenu);
				echo '</pre>';
			}
		}
	}

	add_action('admin_notices', 'debug_admin_menus');
}