Well they usually are born deaf but develop their hearing at about ten days old. But if the cat is white with blue eyes if it isn't a rare type of cat, it is deaf. If one of the parents are deaf...
The cats that tend to be deaf are not just white cats, they are white cats that have blue eyes. They are considered albinos, which in cats can cause deafness.
When your cat has it's back to you clap your hands together really loud. If the cat's ears don't at least flicker then they didn't hear you. You could also drop something on the floor (such as a...