Firstly:
How old is she? Is she white?
•Test. Albinism is common with deaf cats. An albino cat will have pure white (no other coloring, not even one hair) fur, pink skin and blue eyes.
The First Clap Test. When your cat isn't looking, clap once loudly. If the cat reacts, it's not deaf. This test can be magnified if the cat is asleep. If it doesn't keep reading.
Exceptions, older cats who have had to deal with loud noises in their lives may not react to the first clap.
The Second Clap Test. Clap loudly repeatedly. If your cat didn't respond to the first clap, then clap repeatedly for a few seconds. Even a cat who is good at ignoring people will eventually look your way to see what the fuss is about.
The Stomp Test. If your cat does not respond to the clap tests, stomp on the ground. Deaf cats will respond to this test with the same 'what the hell' look as a hearing cat would respond to the clap test.
The Vocal Test. This doesn't mean scream at your cat. Instead, listen for your cat meowing. Deaf cats tend to be either a lot more vocal than other cats OR not very vocal at all. If they are vocal they will not sound like an 'average' cat would sound. They will sound off key.
NOTE: when doing the clap test don't be too close to your cat as she may react due to feeling the vibrations in the air through her fur / whiskers.
Remember: Cats are expert at being deaf when it suits them .... selective hearing is very common. All my gang have perfected the art of totally ignoring me until it suits them to hear. They will ignore me calling them if they are lying in the sun. However, they can hear the fridge door open from the bottom of the garden!