Dehydration has many signs.
The earliest sign of dehydration is thirst. If you're thirsty, you're already dehydrated. Keep a small bottle of water nearby and sip at it periodically.
Another sign is the color of your urine. If it isn't clear, you are dehydrated. If there is any yellow at all, you are dehydrated.
You'd be surprised to learn that the majority of humans are in a constant state of dehydration. Drink more water ;)