My questions are, how do I know that I started to get fatigued? Do I get some kind of message? Or it is not clear when it’s accrue?
And is it confirmed that after 8 hours of playing I will start to get fatigued? And when is the refreshing day that will reset my fatigue back to zero?
I cannot answer for anything except Disciple of War / Magic, and physical experience. And I cannot answer for Fatigue, rather only Surplus experience.
After getting something like 80,000 SP for one class, your experience bar will turn yellow and say "You have gained Conjury bonus experience" / "Your conjury skill bonus rank increased". Then every time you get a skillup, you will get the message at the end of a battle (For each application, so if you got points for healing, buffing, debuffing, and attacking you will get flooded by 4 messages).
And with experience, same thing will happen.
With fatigue, I suspect the bar will flat out turn red instead of yellow
and as I wrote in another post, http://ffxiv.zam.com/forum.html?game=268&mid=1288249639144369859
, I think for DoW/DoM, the system is completely useless and can be completely ignored.