There are electronic scales for sale at places like Target that use both a calculator and electrical impulses to measure your BMI (Body Mass Index) which gives you the fat percentage you're looking for.
Here is a free BMI calculator:
http://www.findmybmi.com/NOTE: If you are a bodybuilder, or are muscular, your BMI done with just a calculator will not be correct. Generally a BMI between 18 and 24 is normal.