functions.php

<?php
**/
 * How to detect is a certain page template is in use.
 * 
 * @link http://www.wupperpiraten.de/2009/12/is-a-certain-page-template-active/
 */
function is_pagetemplate_active( $pagetemplate = '' ) {
    global $wpdb;
    
    if ( empty( $pagetemplate ) )
        return false;
        
    $result = $wpdb->query( $wpdb->prepare( "SELECT meta_key FROM $wpdb->postmeta WHERE meta_key LIKE '_wp_page_template' AND meta_value LIKE %s", $pagetemplate ) );
    
    if ( !$result )
        return false;
        
    return true;
}