With a BMI of 31.7, former NFL linebacker Ray Lewis is well into the classification of "obese". The BMI scale is only useful when accompanied with a picture, and even then it's only useful for saying "hey, that person is proportionately heavy/thin" It is never an indicator of athleticism or internal health, nor is it an indicator of how fat a person is.

There are people who eat healthily and are what would be considered medically healthy, but who are considered overweight or even obese by BMI. There are thin people, even athletic ones with fast metabolisms who don't eat well, and have clogged arteries/high cholesterol, high blood pressure, diabetes, etc.

To answer the question of the thread, body fat percentages are a far more reliable indicator of fatness, since it is a fairly accurate estimate of how much of a person's weight is actually carried in fat. This requires water weighing and is not simple to obtain. I'd say a woman with more than 25% of her body weight being fat, should be considered fat.

For a less scientific estimate you could say if she has a double chin, or if her belly has a hang, but some women don't really put on weight in the face, or don't really develop a belly hang until after they're quite on their way to being fat.

In the words of Potter Stewart, "I know it when I see it."