If you got the ID, and you are auto_incrementing the ID in your database, you can count row before this ID:
$nbRow = $this->Item->find('count', array('conditions' => array('id <=' => $id))) ;
Then to find the page, you just have to divide (int division) by the number of item per page:
$page = ceil($nbRow / 12) ; // 12 or whatever item per page you have