Backup Monitoring

I am trying to figure out the best way to monitor backups to make sure they are completing successfully. I am unable to find a log file that the backups log to. Can someone please shed some light on this or what they may be doing to verify backups are working properly.

With FreePBX 12 you can have a email sent to you when the backup completes or fails

We have a lot of PBXs out there and not all of them will be updated to FreePBX 12. I did find a log under /tmp/backup.log is there anything else or better way other than than this file to monitor this? We are trying to monitor this via a network monitoring tool similar to Nagios.