Reviewing a sample of system-generated backup logs is the best step to verify that regularly scheduled backups are timely and run to completion. Backup logs are records that document the details and results of backup operations, such as the date, time, duration, status, errors, and exceptions. By reviewing a sample of backup logs, the IS auditor can check whether the backups are performed according to the schedule and whether they are completed successfully or not. The other steps do not provide as much evidence or assurance as reviewing backup logs, as they do not show the actual outcome or performance of backup operations.
References: CISA Review Manual, 27th Edition, page 247