Degaussing - Verification

Dear Gurus,

We recently bought a new degausser in order to destroy data on tapes.
This question might sound strange but, my manager wants "somehow" to
verify that, when tapes are returned to customers, he can certainly be
100% sure that there is no single trace of data.  Is there a way to
verify that the data has been completely destroyed?  We've used the 'dd'
command that shows that the number of records IN and OUT is CERO but, my
manager wants to know if there is something beyond that to verify the
existence of any kind of data without destroying or "burning" the tape
itself.  Any ideas are more than welcome. 

Regards,

Orlando