How do you know when a record has been disposed of?

Funny you should ask. A record has been disposed of when it is really gone and you cannot access it anymore. So no copies, no cache and no backup tapes are available and you have no means to reconstruct the record. EU data protection regulators call it “irreversible deletion”.  So get your shredder, burner, eraser out and destroy those documents. It is not sad, it is part of records retention (and life).