Although your body is made up of mostly water, it is important to remember that you can become dehydrated without having obvious symptoms appear.
The best way to know if you're getting enough water is to look at the color of your urine. Pale yellow means you're getting enough. Darker yellow (or amber) means you are NOT getting enough and you need to increase your intake. If your urine is clear, that means you're getting too much.
Keep in mind that some vitamins and other supplements you take, including medications, might alter the color of your urine.
In the olden days, before we knew better, we used to eat a few salty snacks to trigger thirst. But we know now that by the time you are registering thirst, you are already dehydrated. One of my problems is that I will go for a while without drinking and then gulp glasses down at once. Then, I have an extended tummy and look funny!