I'm no expert in this field but on several occasions I have been aware of my own dehydration, and so now am tuned to my own body responses.

I seldom get headaches - but I learned when I was teaching a class all day that I did get headaches. It was a fairly intensive class of 12, I was in airconditioned premises, but I seldom had time for morning tea, lunch or afternoon tea. Sometimes I would manage a cup of coffee! One day I had a particularly bad headache - and it dawned on me. I was dehydrated.

Once I learned to drink more water I eliminatd my headaches, and I have learned since that when I get the feeling/headache a glass or two of water makes it go away. These days I carry a small bottle of water with me all the time.

On another occasion I was badly dehyrated. I was doing a fun run, and I was drinking water along the way. But not enough. I focused on my walking and should have had more to drink. The headache came but worst of all for me, is that my hands swelled. Puffy hands - and my wedding ring started to "choke" my finger. Before I made it to the finish line I increased my drinking of water - but it did take a few hours to resolve.

So, I'd say headaches, and puffy hands as two clues. Obviously if the kydneys have slowed down and one has stopped urinating or at best is producing small amounts of concentrated/dark yellow-brown urine, that is a clue too.