Like many diseases there is no diagnostic test for myalgic encephalomyelitis also known as chronic fatigue syndrome. First other treatable causes of undifferentiated fatigue are eliminated. However ME/CFS is not really about the tiredness that most people experience and should not be confused with the tiredness of depression.
Rather it is characterized by pathological post exertional exhaustion lasting more than 24 hours, unrelieved by rest and upon minimal exertion such as doing the dishes or walking the dog. This symptom is is part of a specific pattern that includes memory/concentration problems far more severe than found in the normal population or in depressed people.
If you don't have this very specific form of fatigue combined with memory/concentration problems the good news is that it is highly likely that you do not have this incurable neuroimmune disease.
For more information go preview.ncbi.nlm.nih.gov back slash pubmed and look up this study:
Myalgic encephalomyelitis: International Consensus Criteria.
Carruthers BM, van de Sande MI, De Meirleir KL, Klimas NG, Broderick G, Mitchell T, Staines D, Powles AC, Speight N, Vallings R, Bateman L, Baumgarten-Austrheim B, Bell DS, Carlo-Stella N, Chia J, Darragh A, Jo D, Lewis D, Light AR, Marshall-Gradisbik S, Mena I, Mikovits JA, Miwa K, Murovska M, Pall ML, Stevens S.
J Intern Med. 2011 Oct;270(4):327-38. doi: 10.1111/j.1365-2796.2011.02428.x. Epub 2011 Aug 22.