Hi, Doglover . . . I was never tested for anything like that until I went to see a naturopathic physician. Not sure all of the tests they ran on my blood but she discovered I was low on B-9 (folic acid) and some of the other main vitamins. I know they can test for magnesium, potassium, iron and B-12 (which I understand are the ones if deficient that can be responsible for fatigue).
Ask your current doctor if they do vitamin and mineral testing. Otherwise, you may want to see a naturopathic physician.
Let us know how it goes and hope you feel better soon!