Cavities are detected a number of ways. The most common are clinical (hands-on) and radiographic (X-ray) examinations. During a dental exam, the dentist uses a handheld instrument called an explorer to probe the tooth surface for cavities. If the explorer “catches,” it means the instrument has found a weak, acid-damaged part of the tooth — a dental cavity. Dentists can also use a visual examination to detect cavities. Teeth that are discolored (usually brown or black) can sometimes indicate a dental cavity.
Dental X-rays, especially check-up or bitewing X-rays, are very useful in finding cavities that are wedged between the teeth or under the gum line. These “hidden” cavities are difficult or impossible to detect visually or with the explorer. In some cases, none of these methods are adequate, and a dentist must use a special disclosing solution to diagnose a suspicious area on a tooth.
Source: http://www.dentistry.com/