Measurement of Water Clarity
Turbidity is measured by the examining the light passing through a sample.  A sample's turbidity is formally measured using standardized methods (e.g. ISO 7027 or "Standard Methods," No. 2130B. source).  Standardized methods require the instrumentation called for by the standard.  

An informal measure of visible differences in turbidity can be obtained by comparing the 20mL sample to a set of calibrated standards.  An example of a set of calibration standards for turbidity are shown in the following link http://www.water.ncsu.edu/watershedss/info/turbid.html.