i cannot really answer about rsnapshot but i use rdiff - similar tool.
there is couple of things i monitor with nagios for my backups:
- what was exit code of last rdiff run. if it's not 0 - it means that something went wrong in the process. i just have simple bash script that executes bacup and depending on result touches file ok or failed. with nagios i check freshness of OK file and alert if it's older then 25h.
- before running backup i touch file in /xyz/touched . on server that receives the backup i check freshness of that file. if it's older then 25h it means last night backup did not arrived to the receiving machine.
- rdiff has verify option. i dont know if rsnapshot has. verify takes lot of time, i run it once a day and put result to a file... then simple nagios plugin greps this log file in search for error messages.
in this way i protect myself against interrupted backups and against file system corruption on receiving server ( for files not modified for long time ).
the trick is not to let nagios plugin do heavy lifting ( eg verifying the whole backup ), but rather schedule cron job that does that and have nagios plugin just checking result of the job ( eg file created or content of log file ).
and by the way - check this out.