How would you know if you have a cavity. my tooth is hurting every time i eat meat.

You can't know: without an examination and possibly an x-ray. But pain in one tooth on eating means something is wrong and cavity is first most likely problem.

See DDS: See your dentist for a clinical and radiographic examination and follow through with the suggested treatment if warranted.

Pain: Pain can indicate infection (abscess), fracture, or damage to supporting tissues. By the time you know you have a cavity it's probably pretty large. Your Dentist can detect small cavities before they become extensive/expensive. Please see a Dentist before your sore tooth becomes an intractable problem.