Check if SQL Agent job is being executed manually

Question

I have a stored procedure that performs database backups. Because I'm looking at Availability Groups within SQL Server 2012 I want to utilise the *sys.fn_hadr_backup_is_preferred_replica* function within that script to test whether the script should produce the backup.

If the script is run manually instead of as part of a scheduled SQL Agent job, or if the SQL Agent job is executed manually rather than via a schedule, then I want the job to fail with an error message to allow the user to see that the backup has not succeeded due to not being on the preferred replica. If the job is run on a schedule then I obviously don't want it to fail as it would be producing errors and sending out alerts all day long on the server that is not the preferred replica.

The only bit that I don't know how to do is to check whether the job is being executed by a schedule or manually. Is this possible?

Failing that, is there any other way that I can alert a user that the script hasn't produced a backup, whilst not causing the scheduled task to fail and produce an alert?