My first sign of fatigue is constant slow down, speed up,,slow down, speed up When I notice that happening I know it's either time for a break or to call it a day. Second sign is trouble keeping visual focus. These things don't normally start showing up till about 8 hours into a full day ride. Here are 3 to read. There's hundreds of articles if you do a search for rider fatigue.
http://motorbikewriter.com/10-tips-beating-rider-fatigue/
https://www.coloradomotorcycleexpo.com/motorcycle-riding-tips-fighting-fatigue-long-trips/
http://www.motorcycle.com/features/motorcycles-hearing-loss.html