It sounds like you are using the offsite backup with incremental backups. I don't use that, so I can't say. I would guess that with the cloud backup, they would be sending verification either through AC or email. That would be enough given you can't test them. You would have to assume they are good backups.
I was referring to the backups I do locally with AC. When they finish they leave a successful backup notification in my inbox. That just tells me that a backup was made and is stored. I pretty much trust it. But, I do test them every once in a while. Except for the changes made in v9 and above, I have found the backups to be bulletproof for over 11 years.
