The Body Mass Index (BMI) measures the percentage of body weight made up of fat, regardless of your build. BMI is calculated as body mass (kg) over height in metres squared. A BMI of more than 25 indicates that you are overweight and a BMI of more than 30 indicates obesity.
It's not very accurate, though. I'm in good shape (moderate to vigorous exercise 4 or 5 times/week), upper body is fairly muscular, but my legs are quite muscular and that pushes me past 25, which say I'm overweight and have moderate health risks. Using BMI, any serious body-builder would be considered obese. Here's a link to an online calculator: http://www.healthlinkusa.com/648ent.htm