try this,
External Module means pure php file outside joomla right ? or its a joomla module ?.
If it is external file like below.
define( '_JEXEC', 1 );
define('JPATH_BASE', dirname(__FILE__) );//this is when we are in the root
define( 'DS', DIRECTORY_SEPARATOR );
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
$mainframe = JFactory::getApplication('site');
$mainframe->initialise();
$session = JFactory::getSession();
$user = JFactory::getUser();
if($user->id > 0)
echo "Logged In";
else
echo "Not logged In";
Its joomla module like
$user = JFactory::getUser();
if($user->id > 0)
echo "Logged In";
else
echo "Not logged In";
Hope it helps..