With the following function you can check if a page is in a certain menu.
/**
* Check if post is in a menu
*
* @param $menu menu name, id, or slug
* @param $object_id int post object id of page
* @return bool true if object is in menu
*/
function is_in_menu( $menu = null, $object_id = null ) {
// Get menu object
$menu_object = wp_get_nav_menu_items( esc_attr( $menu ) );
// Stop if there isn't a menu
if ( ! $menu_object )
return false;
// Get the object_id field out of the menu object
$menu_items = wp_list_pluck( $menu_object, 'object_id' );
// Use the current post if object_id is not specified
if ( ! $object_id ) {
global $post;
$object_id = get_queried_object_id();
}
// Test if the specified page is in the menu or not. return true or false.
return in_array( (int) $object_id, $menu_items );
}