The normal operating temperature of a freezer is zero degrees Fahrenheit or below. Ideally, a freezer should be able to reach minus 10 or minus 20 degrees Fahrenheit to preserve food for as long as possible. Any temperatures above zero degrees could lead to quicker spoilage and loss of quality.
The reason a freezer should operate at temperatures well below the freezing point is because every time the freezer is opened, a blast of warm air enters the appliance. The freezer needs to be able to quickly chill this air before it can begin to thaw out foods, and the colder the freezer, the quicker it can return to safe sub-zero temperatures.
The best way to measure freezer temperatures is through a dedicated freezer thermometer, or by placing a quick-read thermometer between two packages of frozen food.