How to Test a Cat For Deafness
In addition to the methods listed below, may I add a suggestion to fill a metal can full of spare change, pennies, dimes, nickels, quarters, and shake it behind your cat's head to see if he notices. The reason I add this suggestion is that cats absolutely hate the sound that it makes. This was the first test I did on my cat.
The methods below are copied from this link http://www.keoka.com/whtdeaf/advice.htm
Testing for Deafness
Deaf kittens ride on the vacuum cleaner and mew at full volume. When testing for hearing deficiencies, you have to avoid giving any visual clues, What you are looking for is a kitten who can't hear as well as the others, doesn't respond to high pitched sounds or doesn't immediately turn its head towards the source of the sound.
No kitten with any hearing loss has ever got past my battery of tests which start with tearing paper behind their heads and include pop music played through an earpiece.
Kittens are born with their ears and eyes closed. Maine Coon kittens often open their eyes in their first week, but sometimes their ears don't open for three weeks. With both sight and vision, it takes time for the brain to make sense of what the newly opened eyes or ears are telling it, and it can be five weeks before a kitten reacts to sounds. A fear reaction to sounds takes even longer to develop.
For these reasons, I don't do proper hearing tests on kittens until they are 6 weeks old. At that age, you will still not get a big reaction but any movement of the ears will indicate a response. Sometimes it will stiffen up or move its head. As a kitten grows older, it will learn to associate some sounds with danger, and take evasive action. It will also learn to associate other sounds with pleasure, like your voice and food related sounds like cans opening.
Look for a reaction the first time you make a sound. A laid back kitten or cat will recognise a sound which it has decided is not dangerous or food related and may not feel like responding to it again.
Tear a piece of paper behind the kitten's head. Make sure you don't touch it.
Crackle a bit of tinfoil or jingle a bunch of keys. This tests high frequency hearing.
Hiss. This is a universal danger sound. Shield your breath with a tissue or clothe so it can't feel you blowing.
Tap a cardboard box or something that makes a drumming noise to test low frequency hearing.
Run a vacuum cleaner. This makes a wide range of noises, from low rumbling to a high pitched hiss. Do this carefully so as not to frighten them as you will want them to get used to this noise without freaking out. Totally deaf kittens will not react while others will disappear under the furniture. They will sometimes ride on the machine as they like the vibration, but be careful they don't get caught up in it.
Your kitten or cat may have one ear deaf or with reduced hearing. When it is relaxed, throw something small to the left and right of them so they can't see it coming. I use wood litter pellets. The head should turn immediately to the source of the sound. If they have to look around to see what made the noise, they don't have stereo hearing.
This is more technical. I use an earpiece with a small tube attached. I play various sounds down this from pop music to cat miaows at a level which can only be heard when the tube is close to the ear. With this, I can tell, with unequal hearing, which ear is deficient. I've only bred one cat with one deaf ear. He was odd eyed and the coloured eye side ear was the deaf one, contrary to the usual assumption.
(The above comes from this article on the internet: http://www.keoka.com/whtdeaf/advice.htm )