You can open up the Windows Event Viewer and under "Applications and Services\Microsoft\Windows\TaskScheduler", you can right-click on "Operational" and choose "Enable log". That will enabling logging of tasks. Then, when Opera's tasks run, you should see log entries in there under "Operational" (you might have to refresh or retstart the Event Viewer). You should also see the entries on the "History" tab in the properties for the Opera task in the Windows Task Scheduler.
In the log (on the General and Details tabs), you can look to see what it says. If it says just the path to opera_autoupdate.exe, then Arg0 is empty. Otherwise, it should show what Arg0 was replaced with by Opera.
However, in my case, I don't see Arg0 being used (whether the task starts via Windows or when Opera starts and runs the tasks).
When you're done testing, best to right-click "Operational" and clear the log and then right-click it again and choose "disable log" so you don't have a log file building up in size.