You can use $query->getSingleResult(), which will throw an exception if more than one result are found, or if no result is found. (see the related phpdoc here https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/AbstractQuery.php#L791)
There’s also the less famous $query->getOneOrNullResult() which will throw an exception if more than one result are found, and return null if no result is found. (see the related phpdoc here https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/AbstractQuery.php#L752)