I am pretty sure I spent most of my life dehydrated and I didn't even know it. There are still times where I'm dehydrated but at least now I'm aware of it and do something about it.
To put it bluntly; if you pee is the same colour as a character in The Simpsons as opposed to a clear Caspar the Friendly Ghost colour then you are dehydrated.
Other signs that you are dehydrated include:
Dry mouth especially in the morning.
Constipation/ not pooping daily
No tears when you cry
A lot of people don't pee enough and/or don't poop daily. If you aren't peeing at least 7 times a day and not pooping the you are doing somethinng wrong. It's easy to fix though I know from experience.
Whilst I don't know much about the science behind hydration, I can promise you from personal experience that you will become hydrated faster from eating lots of fruit and veg than you will from chugging down actual glasses of water. If you can get your hands on melons or cucumbers you will be on the pigs back.