Yes, it is absolutely possible to diagnose decay in the mouth from a visual exam with a mirror and explorer.  Decay on the biting surfaces of the teeth can often be hiding in a pit, groove or fissure of the outer enamel layer. On a visual exam, the area can look like a dark spot in the white of the enamel or an explorer can get stuck in the cavitation. The x-ray won’t show the decay in this area if is still primarily in the enamel layer. However, once decay spreads past this first layer of defense and lands in the internal dentin layer of the tooth, it will definitely show up on an xray.

Take a look at the following pictures, taken from the same patient in the same appointment:

In reviewing the radiographs, there is no apparent decay in this patient’s molars. There aren’t any suspicious shadows or defects in the enamel and the teeth generally look healthy. 

However, a visual examination reveals the beginnings of a cavity forming in the two maxillary molars. The geography of the tooth allows the decay to be hidden as the three dimensional object is translated into a 2D X-ray. Conversely, not all pit and fissure stains are cavities. Dentists can recognise unique shapes, patterns and texture changes in teeth that differentiate staining from active decay. This further highlights the importance of the entire dental exam to make a proper diagnosis.