At a minimum you need backup, regular restore tests, and alerts when backups stop or restore tests fail.

Personally I automate restore testing with cron. I have a script that picks two random files from the filesystem: an old one (which should be in long term storage) and a new one (should be in the most recent backup run, more or less), and tries restoring them both and comparing md5sums to the live file. I like this for two reasons: 1. it's easy to alert when a cronjob fails, and 2. I always have a handy working snippet for restoring from backups when I inevitably forget how to use the tooling.

IMO alerting is the trickiest part of the whole setup. I've never really gotten that down on my own.