The only way a competent clinician can fail to distinguish CFS from depression is if the patient is outright lying. Most CFS patients for example have exercise intolerance, in which after the patient exercises more than his or her limit (which in severe cases could be walking on level ground for 10 minutes) everything proceeds like it does for a healthy person for 24 hours, then he or she becomes incapacitated for days. Eating lots of carbs, glutamine and creatine will tend to make the period of incapacitation shorter than they would otherwise be at least in non-severe cases of CFS.

My experience (in California) is that a large fraction of doctors are willing to assume the patient is being truthful and another large fraction are willing to spend their time and energy only on patients who can prove they are sick.