During routine visual inspection of a patient's teeth, what is the most common visual sign indicating the presence of occlusal caries?

During visual inspection, dark (brown or black) staining in the pits and fissures of posterior teeth is the most common visible indicator of early or active occlusal caries. The demineralization and bacterial infiltration of fissures causes color changes from the accumulation of chromogenic bacteria, extrinsic stains, and degradation products. The discoloration may range from light brown (early, possibly arrested) to dark brown/black (active progressive). White spot lesions (opaque white areas) are more characteristic of smooth surface caries or early enamel demineralization. Frank cavitation represents advanced caries. Clinical differentiation between stained non-carious fissures and frank caries may require tactile examination, magnification, or laser fluorescence devices.