I ran this question (a similar question) by Bib just to see what he had to say. I’m paraphrasing, but basically he said that he has tried many times over the years to better define and quantify fatigue, but has been unsuccessful. I was talking about hanging BTC and mentioned the feeling of tightness and discomfort in my ligs. He responded that this is exactly the feeling he is talking about and that you know you have reached fatigue on an individual set when this feeling becomes so pronounced that you begin to struggle in your concentration. For instance, having trouble reading, writing, or posting on the forums. Also, as Elvis suggested, when clock-watching becomes intense and you become extremely anxious for the set to be over with the current weight. According to Bib, this is the time to drop the weight for the subsequent set.
A side note: Bib is an awesome dude, I wish he was still here, but having an “ask Bib” forum all its own is a really cool thing as well.