If you have used the same cooking oil before, sniff it before reusing it. If the oil smells off or unusual it has gone rancid. If you are still unsure, heat a few drops of the oil and take a whiff. Another way to test is by checking the color of your UCO. The darker the color of your oil, the more rancid and unhealthy it becomes. DO NOT use oil that smells weird or turned dark brown or even black in color.