Occasionally you need to try restoring from your backups.

Obviously a full restore gives you full confidence, and it goes down from there.

Ideally try to restore about 10% of your content every month,but really it depends on how high stakes your backups are.

Where do you restore to? Do you restore to a spare computer or do you restore into some isolated part (folder) of the production system from which the backup was originally taken?

And to which extent can this be automated, so that the backup gets automatic health checks?

Just use restic. It handles these things.