Here are a few signs of fatigue:

Do you feel weak, as if you are always lacking in energy?
Do you feel exhausted at the end of every day, or even when you�ve just waken up in the morning?
Does your heart beat faster and your breath shorter than usual even with little physical exertion?
Do you often feel light-headed as if you are going to pass out?

If you think you have all these symptoms, then you should see a doctor. Also, re-examine your lifestyle and see what causes unnecessary stress. You may perhaps need to change your diet or work practice. You may also want to be more active and exercise more.