If it makes you violently ill after you eat it, the mayonnaise is spoiled. Flour is spoiled when it wiggles. It never spoils. It is generally a good rule of thumb that cereal should be discarded when it is two years or longer beyond the expiration date. Bibb lettuce is spoiled when you can't get it off the bottom of the vegetable crisper without Comet.
Romaine lettuce is spoiled when it turns liquid.
Any canned goods that have become the size or shape of a softball should be disposed of. A carrot that you can tie a clove hitch in is not fresh. Raisins should not be harder than your teeth. Fresh potatoes do not have roots, branches, or dense, leafy undergrowth. Putting empty containers back into the refrigerator is an old trick, but it only works if you live with someone or have a maid.