There's many different measures of body composition, with the most accurate being either the most expensive or instrusive, hence the popularity of many less expensive and less intrusive forms of measuring body fat percentage.
Types of measure for calculating body fat percentage include:
Air-Displacement Plethysmography (ADP);
Near Infrared Interactance.
My (GP) suggestion would be to perform a 4 site caliper test. Details of which can be found listed under Skinfold Measurements.