How does a dentist know how much to drill without using an x-ray?

X-ray: Ideally anytime a tooth is drilled on an x-ray should be taken. Though it is important, it is not completely necessary since decay is soft the dentist can use a slow drill or an indicating solution to verify the extent of the decay. Hope this helps.

Feel: Teeth are usually hard like any other bone. Soft areas are what we look for as those are the infected areas. Explorers (aka dental picks) are our tool for finding hard or soft areas of a tooth. When we are removing cavities, they cavities also have a different feel than healthy structure. X-rays are for diagnostics, but they only show cavities after a 40-50% loss of density.