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.
sometimes i like that a lot of my work is typical enterprise stuff. nothing gets to prod without some poor soul working through a huge test catalogue on a seperate environment and/or a higher up signs off on it.
it's also annoying because, you cant "just ship" a small fix or change without someone signing off on it.