For many food categories, which sensory indication is arguably the most reliable indicator of spoilage?

Smell.

While the systematic approach of checking sight, smell, and touch is recommended, smell is characterized as arguably the most reliable single indicator for determining spoilage across a wide spectrum of food items. This reliability stems from the fact that decaying microorganisms produce metabolic byproducts that are often volatile compounds responsible for powerful, unpleasant odors such as sour, acrid, or rancid smells. These odors are direct evidence of active microbial breakdown. A food item that registers an unmistakable bad odor should be discarded immediately, regardless of whether its visual appearance or texture seems acceptable, as the chemical signals produced by microbes are often the earliest and strongest warning signs of significant deterioration.