From Dr. Greger’s new book, How Not To Die:

How can you tell if you’re dehydrated? Just ask your body. If you chug some water and pee most of it out soon after, this would be your body’s way of saying it was all topped off. But if you drink a bunch of water and your body keeps most of it, then your tank was running low. Researchers used this concept to develop a formal dehydration assessment tool: Empty your bladder, chug three cups of water, and then see how much you pee an hour later. They determined that if your drink three cups and urinate less than one cup within that hour, there’s a good chance you were dehydrated.¹

¹Novel hydration assessment techniques employing thirst and a water intake challenge in healthy men, Applied Physiology, Nutrition, and Metabolism, 2014