While I have been using Tarsnap for a while now it is first recently I have gotten around to make Nagios monitor those backups. Given that I really don’t want to give the nagios user any actual access to my backups, I instead take the approach of having my backup script create a status file containing a Unix timestamp of the most recent backup.
My check_tarsnap Nagios plugin can then use that status file to check that the most recent backup isn’t older than a specified number of hours. For my nightly backups I have the Warning threshold set to 26 hours and the Critical threshold set to 42 hours.
(See the top comment in the plugin source for an example on how to create the status file.)