How do I know if I'm malnourished?

It is possible to develop some types of nutritional deficiencies if you do not eat healthy balanced meals. In particular, if you are feeling tired all of the time, one possibility would be an iron deficiency. Deficiency in iron is common in those with bad diets, because junk food doesn't contain much iron. Deficiency in iron will cause anemia, which leads to fatigue or tiredness. It is also possible to be tired from the heavy sugar load in junk food. Eat large amounts of simple carbohydrates and sugars at a meals will cause a spike in insulin, which will lead to tiredness after the meal (think of feeling like you need to take a nap after eating lunch, for example). I suggest that you start by going to see your primary care doctor for a full checkup. They will be able to evaluate potential causes of your fatigue, including the potential of anemia. They may want to perform some basic blood tests as part of this evaluation. Based on this evaluation, they will make recommendations on how to proceed. They will also give you advice on how to improve your diet, which is an important step that you will need to take.