I highly recommend the Hidrate Spark water bottle: https://hidratespark.com. It’s not perfect but it’s the most convenient way I’ve found to track water consumption. I’m not affiliated with them in any way.
You can do a blood test for plasma osmolarity and if it out of range there is may be some dehydration. It’s not 100% sensitive according to science, but if it out of range its indicate something in fluid balance
Also there are body scales with bio impedance feature. It doesn’t seems to be accurate for absolute numbers, but good enough to see the trends and shows percent of water in body. Not sure if it can detect small hydration changes, but might worth a try.