Well the first sign is a change in weight. Whether that a visual change in their body appearance or the number on the scale. The healthy weight can be determined depending on the hight and age of the individual. Also, another sign is symptoms of malnutrition. They could feel lethargic, weak, shaky, low energy and even nauseous, but those are mild symptoms. Depending on how long the individual has been not eating enough will determine the amount, severity, or type of symptoms. The reason behind the individual not eating enough is important too, whether it's mental health issues or health problems. If you are trying to determine whether and friend or family member is not eating enough, intentionally or not, you could always try talking to them with a doctor or therapist. I hope this helps!