You cannot tell if a person has anorexia just by his appearance because anorexia also involves mental and behavioral components, not just physical ones. It is not necessary for a person to be underweight to have anorexia. People with large bodies may also have anorexia. However, they are less likely to be diagnosed due to cultural stigma against fat and obesity.
In addition, a person can be underweight without having anorexia. Remember that anorexia also includes psychological and behavioral components, as well as physical ones. People who have anorexia try to keep their weight as low as possible by not eating enough food or exercising too much, or both. This can make them very sick because they start to starve.