How do you tell if you have cavities without going to a dentist?

You generally can't: Many cavities don't hurt, and only show up on x-ray films. Don't wait 'till it hurt, it may not, or by that time you may need root canal!

Simple answer : Typically cavities are not going to be able to be detected without going to a dentist. Most cavities are asymtomatic and won't cause pain until they get large. Brown staining on your teeth may be an indication of a cavity but it's not always the case. Even your dentist would need x-rays to see cavities between the teeth, they aren't seen with the naked eye.

Usually Can't: Unless your cavity is big enough that you can see it with the naked eye, or you can feel it with your tongue, or you're getting food stuck in there, you won't know if you have one. At that time, the hole in the tooth would be pretty big. However, your dentist can find cavities that you can't even see with the unaided eye, especially with x-rays and magnification. That saves you money and pain.