How can my doctor know if anemia is causing my symptoms?

Blood tests. Anemia can cause a number of symptoms from fatigue, cold sensation, shortness of breath, headaches and even chest pains. A blood test will reveal how severe your anemia is, and by comparing the symptoms to the degree of anemia one can make to judgment as to whether the symptoms are likely related to anemia.
Need to be specific. Depends on the specific symptoms, your age and general health, and degree of anemia.