Ops type here, generally you have to fail an AZ and see what happens. If you have Dev/Staging environment that is exactly like production with plenty of tests, you can use that. I've seen only one environment actually like that though.
Since most companies would have to do it in production and they don't want to have customer impacting outage, they tend to do it during an actual outage.
Since most companies would have to do it in production and they don't want to have customer impacting outage, they tend to do it during an actual outage.