The old rule of thumb when it comes to water is 8 glasses a day (2 liters). We now know that this is not a very good guideline. The amount of water you need depends on a number of things. It will vary depending on the individual, the temperature that day, the amount of exercise you got that day etc. Also, if you eat food with a lot of water, like fruits and vegetables, you will not need to drink as much.
The best way to tell if you are getting enough water, or fluids in general, is to pay attention to the color of your urine. You don’t need any special equipment or test kits. Just have a look and if it is very dark and concentrated you are probably not getting enough fluids. You want to see clear to pale yellow.