WordPress is_tree()

is_tree.php

<?php
//usage: place in functions.php

// has a certain page in ID
function is_tree($pid) {  // $pid = The ID of the page we're looking for pages underneath
	global $post;         // load details about this page
	if( is_page() && ( $post->post_parent == $pid || is_page( $pid ) || get_topmost_parent( $post->ID ) == $pid ) ) return true;
	else return false;  // we're elsewhere
}

function get_topmost_parent( $post_id ){
  $parent_id = get_post( $post_id )->post_parent;
  if( $parent_id == 0 ){
    return $post_id;
  } else {
    return get_topmost_parent( $parent_id );
  }
}