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.

Meat

If opening the refrigerator door causes stray animals from a three-block radius to congregate outside your house, the meat is spoiled.

Bread

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.

Lettuce

Lettuce is spoiled when you can't get it off the bottom of the vegetable crisper without cleanser.

Canned Goods

Any canned goods that have become the size or shape of a football should be disposed of. Carefully.

CARROTS

A carrot that you can tie into a knot is not fresh.

Potatoes

Fresh potatoes do not have roots, branches, or dense, leafy undergrowth.

Empty Containers

Putting empty containers back into the refrigerator is an old trick, but it only works if you have a wife.

Unmarked Items

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 your refrigerator to gauge this.