I had occasion to read recently that in addition to urine color, infrequent urination, dark, concentrated urine and/or constipation, signs of not drinking enough water may be:
Fatigue and/or mood swings
Hunger even though you've recently eaten
Back or joint aches
Dull dry skin and/or pronounced wrinkles

I also found out that as one ages, the brain signals that indicate you are thirsty slow down so that you may not realize you are thirsty even though you are, so it is a good idea to keep track of how much you should be drinking especially when recovering from oral cancer and radiation.