Depends on your need, really. If your repo is not too big you simply could run kopia snapshot verify and have it to verify up to 100% of your snapshots. E.g. for our rather big 90T repo a snapshot verify of 5% was done in 15 hrs, so I’d probably not run a 100% very often.
Then we are doing some other teste, where we generate a 10MB file filled with random noise and have it backed up on our regular daily runs. Next day, we have it restored automatically and check the SHA512 checksums against the original one.