Weekly integrity check of all backed up data. E-Mail which informs of result. Web interface which shows overview of results of historical checks. External service which sends E-Mail if integrity check failed to run (e.g. https://deadmanssnitch.com/).
I don't believe it has a web interface with historical checks, although I could be wrong. That said, it might be stored in a log file somewhere.
I also don't have an external service that would send me an email if it failed to run. That said, I would get an email if cronjobs had a mysterious error; otherwise, if the server itself was dead, my data would not be accessible on my home network, so I'd notice.
If the home server dies tragically, well, I hope Google Cloud Storage is doing similar integrity checks -- that's where my offsite backups are.
In .net it's only a few lines of codes. Haven't thought of deadmansnitch but it's a good idea. Would just take one more line of code.
Actually, that reasoning is exactly what I wanted to talk about. When the primary is lost, there is a much higher chance than you'd expect that a secondary contains unrecoverable errors. This is why RAID 5 arrays fail to rebuild after a disk failure so often—they're supposed to be able to tolerate a single disk failing, but they can't tolerate a disk failing and any other IO error at the same time. Part of this is due to how short the timeout is for failed reads in RAID setups, but I've still seen a lot of RAID 5 arrays fail, and I've seen a few RAID 6 arrays fail too.
On top of that, there's the high chance of configuration errors in DIY systems.