Interesting study done on the dangerous effects of drinking too much during races/endurance training. A suggestion was given in the article how to monitor yourself.

From the article:
"Bonci and Almond said a good way to prevent problems is for athletes to weigh themselves before and after training sessions. If they gain significant weight, they should cut back on water intake until they find the right balance — long before race day."

I've primarily relied on thirst alone, but ,I think I'm going to focus on the weight suggestion too. After I empty my bladder first, of course