There must be a hundred different ways to determine if someone is healthy. A general practitioner will check your lung capacity, blood pressure, heart rate, BMI, skin color, hair, eyes, and nails to see if you're healthy.
A psychiatrist will examine your mental and emotional states to see if you are healthy.
Our personal health is a complex thing, but many of us simply rely on a number on the scale to see if we're physically fit.
However, being thin doesn't necessarily mean someone is healthy.