Donnica Moore, MD, President, Sapphire Women’s Health Group, discusses Chronic Fatigue Syndrome and what it is not. She describes in detail how to differentiate CFS from depression, fatigue and other possible medical conditions. 
Taking a good medical history from a patient, such as asking them what they would do if they had more energy, clearly defines CFS versus depression, because the CFS patient will always have a long list of things they would like to be doing but that they cannot. 

Reading: 
Chisholm D, Godfrey E, Ridsdale L, Chalder T, King M, Seed P, Wallace P, Wessely S; Fatigue Trialists’ Group.Chronic fatigue in general practice: economic evaluation of counselling versus cognitive behaviour therapy. Br J Gen Pract. 2001 Jan;51(462):15-8. 
Marshall PS, Watson D, Steinberg P, Cornblatt B, Peterson PK, Callies A, Schenck CH. An assessment of cognitive function and mood in chronic fatigue syndrome. Biol Psychiatry. 1996 Feb 1;39(3):199-206. 
Lane TJ, Manu P, Matthews DA. Depression and somatization in the chronic fatigue syndrome. Am J Med. 1991 Oct;91(4):335-44.