WordPress is_tree function to see if a page is an ancestor.

is_tree.php

<?php
//* Do NOT include the opening php tag

//* Determin if a page is an ancestor 
function is_tree($pid) // $pid = The ID of the page we're looking for pages underneath
{
  global $post; // load details about this page

  $ancestors = get_post_ancestors($post->$pid);
  $root = count($ancestors) - 1;
  $parent = $ancestors[$root];

  if(is_page() && (is_page($pid) || $post->post_parent == $pid || in_array($pid, $ancestors)))
  {
    return true; // we're at the page or at a sub page
  }
  else
  {
    return false; // we're elsewhere
  }
};

usage.php

<?php
//* Do NOT include the opening php tag

if (is_tree(2)) // 2 is the id of the page we are looking under
{
  // stuff
}