How to determine if Update statement was successful

I know this has a simple answer that I should know, but for the life of me, can't remember. I have the following update statement in my code:

$stmt = $pdo->prepare("UPDATE `files` SET `filename` = ?, `md5` = ?, `filesize` = ?,`logtime` = now() WHERE `id` = ?");
$stmt->execute (array($isoname, $md5file, $isosize2, 1)) ;

How do I test if the update was successful?