There are two methods of diagnosing excess storage fat.
The first method is inspection whereby an excessive amount of storage fat is usually noticeable upon visual inspection.
The second method is inference of body fat content, obtained from body measurements such as weight, and comparison with charts of similar measurements in broad populations.
The determination of excess fat is based on the amount of variance from "normal," a value that comes from statistics on death rates in people with similar measurements.
You can also get an estimate of body fat by using our body fat calculator.