A sure way to gauge how hydrated, or dehydrated you are, is to monitor the darkness and clarity of your urine.
If your urine is dark in color, and/or not very clear in opacity, it could be, and likely is a sign that you are somewhat dehydrated.
It's recommended that you drink 2 liters of water, on average, a day, but everyone is different. I suggest listening to your body, and perhaps creating a hydration schedule for yourself to help stay hydrated.
If you become lethargic, and faint, you may be extremely dehydrated, and should seek medical assistance if this is the case.
Also, be careful of over hydration !! Yes it is possible to drink too much, and is very dangerous!
For your convenience, here is an article about over hydration:
https://en.wikipedia.org/wiki/Hypernatremia
I hope this helps answer your question!