Chronic fatique syndrome symptoms??

What are the symptoms of chronic fatigue syndrome? How is it diagnosed? I am asking because no matter how much I sleep... I never feel rested. I am constantly tired, have NO energy, I literally feel like a zombie going through my day... I continuously catch myself staring off into space, etc. And I am often "forced" to take take naps during the day (gets to the point where I can't hold my eyes open or my head up)... but when I do take naps I sleep for so long that I don't know if it would still be considered a nap (I can sleep for like 5-6 hours at a time).... here recently, I have had days where I sleep for like 20 hours on and off (meaning I might get up and only stay up for maybe minutes at a time)