You're probably familiar with what rancid oil is, but you might be less familiar with what it smells like. When checking oils of fatty foods for rancidity, sniff for metallic, bitter or soapy aromas. Photo James Jordan.
Still not sure? Touch the bottle. Sticky residue is a sign of advanced rancidity, and could indicate the contents inside are starting to turn on you. You can also pour out a little oil between your fingers and check for tackiness. If it feels sticky, toss it.
Click the link below for a more complete explanation of the process, but enzymes, oxygen and heat all play a role in driving rancidity, and technically any fat-containing food can go rancid. Vegetable and olive oils are particularly susceptible, and you need to be even more watchful with those that have already seen some heat, like toasted sesame, so keep those in the fridge for extra protection. Fats that are solids at room temperature are more chemically stable, and less likely to go rancid, but I'd give 'em a sniff every once in a while to be sure.
How to Tell If Your Food Is Rancid (Ew)| Epicurious