If it makes you violently ill after you eat it, the mayonnaise is spoiled. This is NOT a marketing ploy to encourage you to throw away perfectly good food so that you'll spend more on groceries. Perhaps you'd benefit by having a calendar in your kitchen. If opening the refrigerator door causes stray animals from a three-block radius to congregate outside your house, the meat is spoiled. Fuzzy and hairy looking white or green growth areas are a good indication that your bread has turned into a pharmaceutical laboratory experiment.
Romaine lettuce is spoiled when it turns liquid.
A carrot that you can tie a clove hitch in is not fresh. If it looks like it is ready for planting, toss it. If you can take it out of its container and bounce it on the floor, it has gone bad. Putting empty containers back into the refrigerator is an old trick, but it only works if you live with someone or have a maid. Generally speaking, Tupperware containers should not burp when you open them.