By taking a careful dietary history. You must ask about the type of food, amount of food and frequency of feeds. If the diet appears to be good according to the mother’s history, consider a disease such as chronic diarrhoea or infection as the cause of the child’s poor nutritional state. Many illnesses can lead to malnutrition, e.g. measles. Sometimes, only a response to a good diet confirms the diagnosis of malnutrition due to a poor diet.