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 looks 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 already is.

Mayonnaise - If it makes you violently ill after you eat it, mayonnaise is spoiled.

Frozen Foods - Frozen foods that have become an integral part of the defrosting program 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 kitchen door, the meat is spoiled.

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

Canned Goods - Any canned good that has become the shape or size of a basketball should be disposed of carefully.

Carrots - A carrot that you can tie in 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.