Answer: What you should do is drink when you are thirsty. There is no need to count the glasses and to try to figure out by mathematical analysis whether you have consumed enough water. If you need more water, you will feel thirsty, and then you can drink. It works quite simply. But if you need a number, I would estimate that the average person should have four glasses of water per day.
Answer: Observing: Look at the water, if you can see anything gross, such as poo or medical supplies, the water is unsafe to drink.
Experimenting: Drink some of the water, if several hours later you experience what can only be described as an epic evacuation of your gastro-intestinal tract, chances are the water was NOT safe to drink.
Modeling: Be sure to take pictures of your epic diarrhea and projectile vomiting session to show others the dangers of drinking polluted water.