The function hasFailedTrans() reports if a smart transaction has failed, or if the failtrans method has been called. The method must be called before completetrans, otherwise it will not report correctly.

$db->startTrans()
/*
 * Table updates, correct?
 */
if ($db->hasFailedTrans())
{
    /*
     * Do something
     */
}
$db->completeTrans();