BMI is pretty straight forward: BMI=703*(weight/height^2) where weight is in lbs and height is in inches. Although they say it's to measure "body fat", it's no better measurement than weight, since it still doesn't differentiate between muscle and fat.

Probably the best indicator is body fat percentage, but the ones that measure electrical resistance (which you can do at home with a special scale) are pretty inaccurate and it's pretty big bucks for a submersion test.