[tomatocracy]

I'm sure there will be plenty of advice for testing the backups themselves, but for online backups (either to server or to a service), one thing which is easy to forget to do is to check you can still get access from "scratch" - ie if you have lost all local data. Make sure you don't need eg passwords or keys for the online backup service which you have no other copy of (doesn't help if they are in the backup itself!) or indeed for email/cloud accounts which you might need to regain access to to get to your backups.