is_tree.php

<?php
/* Place this within the themes functions.php file */
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))) 
               return true;   // we're at the page or at a sub page
	else 
               return false;  // we're elsewhere
};

/* Use this within the page templates */
if (is_tree(2)) {
   // stuff
}