The managed OPA instances are automatically configured to export Prometheus metrics. See operators:monitoring.adoc for more details.
You can read the list of exported metrics in the OPA monitoring documentation.
Especially worth mentioning are the Status metrics, which contain e.g. opa_info, bundle_loaded_counter and bundle_failed_load_counter.
You can use these to create a dashboard to see any bundle loading problems at a glance.
You can also set up alerts to get notified proactively.