I have Congestive Heart Failure, and fluid buildup is not my friend. I weigh myself on a digital scale, usually at least twice daily, after urinating. I find my body fluctuates through the day by about 3-5 pounds, at least partly due to fluid intake and retention. When I lose too much, I get light headed (dizziness, sometimes profound) if I get up onto my feet too fast. If I retain too much, it constricts my chest so getting a full breath can be difficult.
I asked my VA Primary Care Physician about drinking extra water, and his response was that nobody had yet convinced him that extra water intake has real benefit, and that in my case, with all those blood pressure and diuretic meds my cardiac condition entails, I would do well to avoid 'common wisdom' on the subject. I've already been warned that there is some evidence that kidney failure is beginning to appear, probably triggered by the meds I need to take. Because of those meds, a dry mouth is not a reliable indicator of dehydration, and I need to apply a fluoride gel before bed because of nighttime dry mouth and its propensity for encouraging tooth decay. At least I still have most of them...
Nowadays, when I meet friend for drinks at the VFW, I often drink Cranberry Juice instead of beer.
Checking my weight, reviewing that in comparison to the darkness of my urine, is my method. Enough color to see it, but not darker. At my age (67), fluids after dinner just mean I will have to get up at least once in the night.
Greg