Usually you can figure out on your own if a cat is deaf. Place the cat in a quiet room without any stimulans and then make different kind of noises. Make sure the cat doesn't see you making noises.

If you're still unsure after testing on your own you can have a vet do a BAER-test. That'll tell for sure if the kitten is deaf or not. A BAER-test however won't tell you if the hearing only is reduced. It'll only tell you if the kitten hears or not.

I've raised a deaf kitten and we saw signs of deafness as early as when he was only 4 weeks old. At 8 weeks of age we were completely sure he was deaf. Never had to do a BAER-test to confirm it.