I, too, have a deaf ferret.  He has mostly a white head with silver streaks
just above his ears giving the appearance of a white streak along his head.
He also cannot hear the vacuum.  The first test I did was take a pot and a
spoon and bang it loudly while he is sleeping.  Do it away from the ferret,
across the room, so he will not sense the vibrations.
 
The one time I did that, my Domino did not stir but the other three woke up
looking rather disturbed.
 
Also, if he vocalizes more than other ferrets (i.e., his dooks or war cries
or sounds of hurt), then he is probably deaf.  Deaf ferrets, along with
deaf people, do not know how loud they are.
 
Not that there is any correlation, but my deaf ferret is the most active
ferret I have ever known.  He brought new life to the other older ferts.
 
Matt and Domino "What's to hear?  I get along just being cute."
[Posted in FML issue 2611]