Water pollution can be measured in a very scientific way. Physically, the testing measures include taking the temperature of the water. The total suspensian of the particals in the sample of water tell the tale with their turbidity. Then, chemicals are measured and detected by way of testing the measurement of the pH value.
These tests are called Biological Oxygen Demand and Chemical Oxygen Demand. The same tests are done on swimming pools and spas to determine many factors. Another way the water's purity can be assessed is by taking a good look at the life within it.
If the plants and animals and fish that are in it or that drink from it or swim in it are having issues, chances are good the water is polluted. When a bunch of fish die all at one time in one place, for instance, it is time to test the water a little deeper.