To monitor the NSX Manager backup, VMware recommends to use VMware vRealize Log Insight which is available to use with NSX for vSphere license.
After configuring IP of Log Insight in NSX Manager's management interface, NSX Manager sends out the logs to remote Syslog(Log Insight) in the case of success and failure:
Sample output:
Success:
Jul 13 02:08:48 xxxxxx-01a [SystemEvent] Time:'Thu Jul 13 02:08:48.442 UTC 2017', Severity:'Informational', Event Source:'Take Backup', Code:'9009', Event Message:'Backup of vShield Manager successfully completed.', Module:'vShield DB Backup Restore'
Failure:
Jul 13 02:02:24 xxxxxx-01a [SystemEvent] Time:'Thu Jul 13 02:02:24.971 UTC 2017', Severity:'Critical', Event Source:'Take Backup', Code:'9006', Event Message:'Backup of vShield Manager failed.', Module:'vShield DB Backup Restore'
After you identify these logs in Log Insight, select the Event Message verbiage highlighted for the failure case. You can also setup a Log Insight alert to send an email alert if you see this message.
This provides the ability to take corrective action in the case of NSX Manager backup failures.