Know yourself: Before going out on your training runs, step on the scale, physician Bob Murray advises. Then, step on again when you're done training. Gain weight, and you should cut your fluid intake, he said. If you lose more than a couple of pounds, take in a little more fluid the next time.
"If you do it often enough during training, it really becomes second nature, and you have a personal sense of how much fluid you need," Murray said.
Know your symptoms: Pay attention to a headache that won't go away, vomiting and confusion. It will be hard for you to tell whether it's overhydration or dehydration, so seek help from a doctor, Murray said.
"Most people within two hours are feeling a lot better, and if you're not, you want to go to the emergency room," physician David Bernhardt said.