And no, I have not tested it because I don’t know how I’m actually supposed to do that.
depends on what you backup and how.
if it’s just “dumb” files (videos, music pictures etc.), just retrieve them from your backups and check if you can open the files.
complex stuff? probably try to rebuild the complex stuff from a backup and check if it works as expected and is in the state you expect it to be in. how to do that really depends on the complex stuff.
i’d guess for most people it’s enough to make sure to backup dumb files and configurations, so they can rebuild their stuff rather than being able to restore a complex system in exactly the same state it was in before bad things happened.
agpl does not “steal” sales, but i have to give my users the source code under a gpl compatible license, that includes that they redistirbute the code however they see fit.
that scares many people, but i guess they forget that your game is more than code and the license does not cover assets