One way is to use a device that does it:
Amazon.com: Omron Body Fat Analyzer: Sports & Outdoors
Or use body measurements:
Body Fat Calculator
Body Fat and Skinfold Thickness - How to Calculate Body Fat Percentage from Skinfold Thickness
The immersion method (most accurate):
Body fat testing - hydrostatic body fat testing - body fat equipment: FitnessWave (http://www.getdunked.com/home/default.asp - broken link)
Some gyms will do it for members with either a device or do the measurements for you.
There may be variation in the absolute numbers obtained with any method, but they are useful if you just want to confirm that you are losing fat by comparing serial measurements.