Decay may or may not cause discomfort; even though it doesn't hurt, the tooth is deteriorating.
Using higher magnification and powerful lighting, it is easier to detect decay at an early stage to prevent excessive tooth damage. When cavities are small, they are much easier and less expensive to treat.
Early tooth decay does not tend to show many physical signs. Sometimes the tooth looks healthy, but your dentist will be able to see from an x-ray whether you have any decay under the enamel, any possible infections in the roots, or any bone loss around the tooth.