Visual clues include changes in color and viscosity, the presence of impurities, and a lowered smoke point. An oil perceptibly darkens with use - and especially with misuse - because the molecules of both oil and food burn when subjected to high or prolonged heat. Because sugar caramelizes, sugar-rich foods can particularly deepen the oil's color.
The more you use an oil, the more slowly it will pour. This decrease in fluidity is mainly caused by an alteration of the oil's molecular structure, but also by the accumulation of loose, absorbent food particles, which can be seen as sediment or suspended flecks. When smoke appears on the oil's surface before the temperature reaches 375 degrees F, your oil will no longer deep-fry effectively.
An oil's downfall is also indicated by rancidity, primarily caused by prolonged contact with air. An oil's rancidity can best be described in sensory terms by comparing it with the taste and smell of a stale potato chip. If the oil's natural odor and flavor have been unduly contaminated by foods cooked in it, oil should be discarded. The same is true if it tastes burnt or inordinately oily.