How can I tell if my RMAN backups are using synchronous or asynchronous communication?

Hello, I am trying to tune the RMAN performance in my environment.  The documentation for RMAN tuning talks about v$backup_sync_io and v$backup_async_io and seems to say that generally, synchronous I/O is evil, and asynchronous is not.  But how can I tell which I am using, and how can I control it?

Thanks,