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. Flour is spoiled when it wiggles. It never spoils. 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.