Is it possible to tell the source of a backup? I was wondering if it is possible to tell the difference between backups created manually and backups created through a maintenance plan.

There does not seem to be any way to discriminate them in the msdb backup related tables apart from the account that the Sql Agent runs under is placed in the user_name column in the msdb..backupset table

I would like to verify that backups done through a maintenance plan exist on disk but don't really care about any manually created ones. Part of the plan compresses and copies the archives to another server. These will be compared to make sure everything is ok with the plan.
When you set up a backup using the Maintenance plan, it provides a rather complex name for the database backup (example: MovieManagement_backup_2012_02_14_064551_7520824). You can view this by using RESTORE HEADERONLY. Most people don't supply any name at all when running a backup manually. This can be the difference. Other than that, no, I'm not so sure there's anything you can do there.