Oracle DataGuard: Find primary from standby DB
You can check some options like checking FAL_SERVER parameter, LOG_ARCHIVE_DEST_n parameters, Dataguard Broker, Recovery Catalog, Cloud Control, etc..  if they are used, but none of them are mandatory for a DataGuard configuration.

In fact, the standby nodes doesn't need to send any kind of information to the Primary db at all. Since the heartbeat is checked from the Primary, the best approach is to query v$session and track down sessions connected as the, non-existent, "PUBLIC" schema.