Just recently Dr. Kawasaki here in Virginia has been working researching
a degenerative eyesight problem in ferrets.  He believes that there is a
degenertive loss of sight that can be seen be tracking the loss of blood
vessels in the eyes.  The affected ferrets would have less visible blood
vessels in the back of the eye than normal.  A vet who has seen the eyes
of many normal ferrets should be able to detect the lack of them I would
think.  This though isn't the only reason ferrets can go blind unfortunately.
 
I can't come up with a truly reliable way to test either hearing or sight
in ferrets myself.  They can ignore you with the best of them and then
react to the least thing.  Gotta love them though.
 
bill killian
zen and the art of ferrets
[Posted in FML issue 1314]