Simple Check for a logged in session:

<?PHP
if(ISSET($_SESSION['user_name']))
{
// Don´t perform any action cause a Logged in session is detected
} else {
// Send the snooping people back that has not logged in
header("Location: index.php");
exit;
}
?>

__________________
Of course the whole point of a doomsday machine, would have been lost if you keep it a secret.