Couldn't really guess, I only directly monitor intake when I'm at risk.



Headaches, I always know to lean back on the coffee and drink more water then. Bad-taste in mouth, urine color, stitch in the side, are regular warnings if I have other reasons for a headache or took pills.



Yes, it's very variable.



Gave up the sauce for lent but I've found pouring a glass of water every time I fixed a drink and not pouring another drink till I finished both is ideal for maintaining hydration, avoiding hangovers, and actually tends to help cut down on intake.



If you only had to use the bathroom once after consuming that much, you were likely real bad off, it takes a few days of heavy hydration to really build back up to full. On the other hand, it entirely depends on what you were doing. I remember when we were taking a census of a village in Anbar I'd been well hydrated before hand, and when we left for the mission I'd been regretting not hitting the latrine before we rolled out. I drank roughly 6 liters during the five hour mission, mid-day of august, never took a whiz, didn't need to when we got back or for some time afterwards even though I drank a couple more liters and a whole pot of coffee when we did. So it entirely depends on what you were doing. You mentioned a gym but that dpends a lot n what you're doing, weights and even quick cardio don't always use much water, whereas fat-burning on a treadmill or an elliptical for an hour can seriously suck up water even though your heart never goes over 130-140, so it depends on what you were doing. If your sweat doesn't really smell much your usually well-hydrated and all is well, if you reek something awful, then typically you're not.



When in doubt drink lots of water and eat more soup for a few days, that's about it. If you're drinking much coffee or alcohol regularly then yes. Less so for soda, but it's a diuretic too.

Remember that it takes 3 things to burn fat, air, water, and a reason for your body to initiate burning. So during exercise you can really chew through water and if you aren't well-hydrated you are getting minimal results for you pain. Less of a factor for muscle building but in long workouts if your body doesn't have the water to burn fat quickly you can't put the muscle on because your body will start burning it for energy. So never work out if you're not well hydrated.