Turbidity is a function of the dissolved particles in water and the color of the water. Clean water is transparent, so water that has any coloration is contaminated with something in all cases. The dissolved particles will typically be sand, algae, or clay, but there could be variations. The turbidity of water once had to be measured in a purely qualitative sense.
People would just try to rate the turbidity levels of a given quantity of water in a purely subjective manner. Most people tended to avoid water with an apparently high turbidity level, even though turbid water is not necessarily as dangerous as people might think.
There are turbidity meters and turbidity sensors that people can use today. These tools will tend to make the process of measuring turbidity relatively simple. The water quality monitoring professionals will just have to make sure that they are using the tools correctly.