Some of all 3 plus experience. One of the benefits of secchi readings is to watch the changes over time. A lot of it is common sense. If there has been no big rain lately then its probably not silt. If a new pond with not much veg ( in the pond or watershed) to decay then not humic. If water is warming and greening is occurring then probably plankton bloom. The scientific process is always about observation , hypothesis and testing. Over time and observation you get a sense of if it is colloidal suspension , veg decay or plankton. What type of info would help you ?