WordPress does not include a function to check if is_subpage. This function provides that for us.
// This function will return true if we are looking at the page in question or one of its sub pages
function is_tree( $pid ) { // $pid = The ID of the page we're looking for pages underneath
global $post; // load details about this page
if ( is_page($pid) )
return TRUE; // we're at the page or at a sub page
$anc = get_post_ancestors( $post->ID );
foreach ( $anc as $ancestor ) {
if( is_page() && $ancestor == $pid ) {
return TRUE;
}
}
return FALSE; // we aren't at the page, and the page is not an ancestor
}