What does flour smell like when it goes bad?
The smell is of rancid fats in the flour. It is a very distinct smell, but hard to describe. I would agree that "sour" is part of it. The best way to learn is by comparison. Smell some fresh, clean flour and remember what it is like. Any kind of unpleasant odor than that is rancidity. You may have smelled it also if you opened a box of cereal that has been around too long, or even poppy or sesame seeds - anything with fats that sits around at room temp will eventually go rancid.