Since we know that the role of an auditor is not to detect fraud, how can the auditor perform his tasks to make sure that fraud are not perpetrated?

every financial transactions includes fraud factors by some conditions or situations which is lead to commit to fraud..
as a auditor we can do the following steps for: depute a person to do indebt verification,check the accounting policies specifically related to the accounting assumptions ,application of unpredictable audit procedures and so on once we discovered the fraud we must report to top authoriy

What an auditor can do to make sure that fraud are not perpetrated is to constantly review the internal control system of the organisation and test it to see that the internal control is sufficient to prevent the perpetration of fraud.