WordPress at a glance
function is not described

is_nav_menu_item() WP 3.0.0

Determines whether the given ID is a nav menu item.

No Hooks.

Return

true/false. Whether the given ID is that of a nav menu item.

Usage

is_nav_menu_item( $menu_item_id );
$menu_item_id(int)
The ID of the potential nav menu item.
Default: 0

Code of is nav menu item: wp-includes/nav-menu.php VER 5.0.3

<?php
function is_nav_menu_item( $menu_item_id = 0 ) {
	return ( ! is_wp_error( $menu_item_id ) && ( 'nav_menu_item' == get_post_type( $menu_item_id ) ) );
}