Do dentists write down exactly where each cavity is on a tooth?

Yes, Dentist's record where your problem area's are. Sometimes, cavities are visual, (on the surface). Othertimes, cavities are inside your tooth. Dentist's take X-ray's of your mouth, and based on these X-ray's they can pin point exactly where your problems are.

There are 5 surfaces on a tooth, and one or more can be affected by a cavity. That is why it is very important to write down on a dental chart where the cavities are located on each tooth, and all dentists generally do it.