Some mistakenly believe that oils do not spoil, but the truth is that they have a shelf life like any other food product. It is possible to tell that cooking oil is rotten if it is sharp, bitter, smells bad like old paint, nail polish remover, or smells different from its original smell.
The smell may not be a clear indication of spoilage, while the taste is so because the taste of vegetable oil without any added flavors is neutral and closer to tasteless. Some oils have a longer shelf life than others due to their chemical composition, but storage conditions and the way the oil is kept can also make a big difference in the quality and life of the oil.