The first is waist to hips ratio. Especially in women a recent report has shown that a high waist to hips ratio is a powerful predictor of heart disease. Second is the size of your waist. In particular a waist of 35 in women and 40 in men predicts poor health. Finally a high body fat percentage is predictive of poor health. Body fat can be measured by infra-red devices, calipers, underwater weighing and electrical impedance (the method we use at our office).
In short, a person with a noticeable belly is far less likely to be healthy than someone with a flat stomach. And stomachs only become flat by weight loss, no amount of situps or other exercises for “spot reduction” will work. The preceeding is not to say that BMI should be ignored, just that it is possible to have a higher BMI but a low body fat percentage. If you are unusually muscular a slightly higher BMI may not mean that you are likely to develop poor health.
Overall though being lean is the most important controllable variable to attain good health.