There are many standards to assess water quality with. However, the most common standard used, relates to the health of ecosystems, human contact safety and drinking safety. Here are some ways we test water!
Turbidity: Sediment makes water turbid or unclear. Clear water is of better quality. This is tested simply by looking at the water.
Temperature: Warm water has less dissolved oxygen than cold water. If the water does not contain enough oxygen, aquatic animals may die. This can be tested with a thermometer.
Acidity: Water becomes acidic if it contains certain dissolved chemicals, such as sulfuric acid or nitric acid. Most fish cannot reproduce in acidic water. This can be tested with an indicator that turns into different colours in acidic and basic solutions.
Hannah R.L. - Water Systems 