THE GAG TEST
Anything that makes you gag is spoiled (except for leftovers from what you cooked for yourself last night).
EGGS
When something starts pecking its way out of the shell, the egg is probably past its prime.
DAIRY PRODUCTS
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.
MAYONNAISE
If it makes you violently ill after you eat it, then the mayonnaise is spoiled.
FROZEN FOODS
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.
MEAT
If opening the refrigerator door causes stray animals from a three-block radius to congregate outside your house, the meat is spoiled. If opening the refrigerator door causes stray animals from a three-block radius to leave the neighborhood, the meat is really spoiled.
LETTUCE
Bibb lettuce is spoiled when you can't get it off the bottom of the vegetable crisper without Comet.
CANNED GOODS
Any canned goods that have become the size or shape of a basketball should be disposed of. Carefully.
CARROTS
A carrot with which you can tie a clove hitch is not fresh.
WINE
It should not taste like salad dressing.
POTATOES
Fresh potatoes do not have roots, branches, or dense, leafy undergrowth.
CHIP DIP
If you can take it out of its container and bounce it on the floor, it has gone bad.
GENERAL RULE OF THUMB:
Most food cannot be kept longer than the average life span of a hamster. Keep a hamster in your refrigerator to gauge this.