Teeth are made up of layers. The outside is the enamel layer and is very hard, the next layer is the dentin and it is softer, in the center is the pulp; this is where the blood vessels and nerves are located. Your dentist uses different procedures for diagnosing cavities in your teeth, these include a visual exam, images including x-rays, and instruments that detect deterioration in the enamel layer of the teeth.
The initial stage of a cavity is a weakening in the enamel layer, and may be reversible if it has not passed through to the dentin. The enamel at this point may have a chalky appearance and there may not be any sensitivity.
Once a cavity is through the enamel and into the dentin it will grow and damage tooth structure quickly. Often a cavity at these first stages will not cause a patient any discomfort. As the cavity grows and approaches the pulp, symptoms may start. A cavity or fracture that is large enough to affect the pulp cannot be repaired with a filling alone. A filling can be used to stop the growth of a cavity, to rebuild the portion of tooth structure that has been lost, and to restore function and esthetics.