How to find the callback handler for a URL

The manu system handles callbacks in Drupal so you need to look into the menu_execute_active_handler() function.

For a 5.x solution look here

For 6.x here is the fragment for adding the callback information to the error log..

  if ($router_item['access']) {     
    if ($router_item['file']) {
      require_once($router_item['file']);
    }
    error_log("===>  URI Path: \"$router_item[href]\" is handled by: $router_item[page_callback]()");
    return call_user_func_array($router_item['page_callback'], $router_item['page_arguments']);
  }