My experience was that there is no firm way of diagnosing CFS. For 18 months I had every known test, all of which told me I was healthy, and in the end I was told: We think you have CFS. I looked at CFS on the internet, and found that I had nearly all the symptoms.
Blood tests found nothing, in my case, and almost all the doctors I saw had little idea of CFS, and tried to tell me it was all in my head. I have, as a result, little faith left in mainstream medicine.
There are, however, doctors who specialise in CFS (though not where I live) and make sure, before you spend money on a doctor, that you have found one who understands CFS: many of them do not accept that it exists, since they have no sure test for it and no available cure.