Well, one way could be testing all fresh files + some old randomly.
Anyway, what then if test shows that something is wrong? There’s pretty much nothing you can do about that. Because Duplicati still fails with latest canary. All repair options are more or less broken. Test, Repair, List-Broken-Files and Purge-Broken-Files. Test fails, but there’s no way to remedy the situation, other than deleting everything and starting backup set from clean slate. Restore keeps failing, even if backing up the data shows deceptively that it’s all good. - Program logic is still very seriously lacking and restore fails.
Serious things like this, are the exact reason why the software shouldn’t be used in production. It won’t work reliably or in a sane way from logic / integrity point of view. -> Very dangerous and deceptive to use. -> All efforts and resources used to create backups, which can’t be restored, are absolutely and literally wasted energy.
I’m forced to completely recreate the 264 GB data backup set which was over 100 GB as Duplicati files, because those are unrecoverable due to bad software logic. Probably very small operations would have been enough to fix the backup set, but that logic is missing from the software. Or I could have manually try to delete more files, but that would have probably been uncertain outcome. These are exactly the things which shouldn’t be manually handled.