So, you've just gotten a new ferret. If your ferret jumps when you go to touch it (or bites out of surprise when touched from behind or when woken up), has Waardensburg markers, or does not seem to respond to stimuli such as chirping toys which, at the very least, draw most ferret's attention even if only for a second, then you should test the animal for deafness.
TESTING FOR DEAFNESS:
When testing for deafness, it is important test the animal more than once - repeat the test at different times to ensure that you don't get a false positive or negative. Keep in mind that deaf ferrets have learned to cope with deafness and will often follow the lead of ferrets that can hear, so it is best to test your ferret's hearing either on its own or with a deaf ferret to decrease the likelihood of false responses.
1) When your ferret is not paying attention to you, take a noisemaker - pennies or marbles in a jar work well - and shake it behind the ferret. The ferret, if it can hear, should look at you or dart away and peer our at you from under a couch. Be sure that the ferret does not see you shaking the jar as this may illicit a false response - the ferret will get excited or look at you to see what you are up to.
2) Repeat the test with other noisemakers and at different times. If your ferret is deaf, the response will be the same each time - he or she will continue on with what they are doing (unless they see you shaking a noise maker, at which case they will more than likely see you shaking an object and begin to get excited)
3) Squeaky toys can be used for testing, HOWEVER, be aware that some ferrets that are have Waardensburg syndrome are not stone deaf and may be able to hear certain frequencies. These ferrets should be treated as deaf because, for all intents and purposes, they are. They may be able to hear a squeak, but will not be able to hear you approach them when they are asleep.