The Gag Test
Anything that makes you gag is spoiled (except for leftovers from what you cooked for yourself last night).
When something starts pecking its way out of the shell, the egg is probably past its prime.
Milk is spoiled when it starts to look like yogurt.
Yogurt is spoiled when it starts to look like cottage cheese.
Cottage cheese is spoiled when it starts to look like regular cheese.
Regular cheese is nothing but spoiled milk anyway and can't get any more spoiled than it is already.
Cheddar cheese is spoiled when you think it is blue cheese but you realize you've never purchased that kind.
If it makes you violently ill after you eat it, the mayonnaise is spoiled.
Frozen foods that have become an integral part of the defrosting problem in your freezer compartment will probably be spoiled - (or wrecked anyway) by the time you pry them out with a kitchen knife.
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 calender 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.
Sesame seeds and Poppy seeds are the only officially acceptable "spots" that should be seen on the surface of any loaf of bread. 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.
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. Carefully.
A carrot that you can tie in a clove hitch is not fresh.
Raisins should not be harder than your teeth.
Fresh potatoes do not have roots, branches, or dense, leafy undergrowth.
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.
You know it is well beyond prime when you're tempted to discard the Tupperware along with the food. Generally speaking, Tupperware containers should not burp when you open them.
General Rule Of Thumb
Most food cannot be kept longer than the average life span of a hamster. Keep a hamster in or nearby your refrigerator to gauge this.