https://fba.ryona.agency/ is one website that can help, top search box searches for instances that have defederated from the one you entered, the bottom one lists the instances the one you entered defederated from.
It only shows full instance level defederation, not blocked communities though
I just automated syncing daily and sending myself an email with the emerge -pvuDU world output. I usually actually run the update once or twice a week.