How can you tell if u have a cavity without going to the dentist?

Generally you can't: It's possible you'd have pain in the tooth or sensitivity to cold/warm. However, it's best to catch a cavity early before it does more damage. Also, sometimes the nerve can become damaged and loose sensation from a cavity and then you won't have pain. Best idea is to see your dentist regularly.

So many variables: Cavity diagnosis without the help of a dentist is like making any other medical diagnosis without the help of a doctor. See your dentist.