That is a very good and fair question.How do you confirm what caused the problem without having accurate water test results?
Inasmuch as plaster (cementitious material) is directly affected by the water balance (CSI), one can study the plaster surface and obtain a great deal of information without having and knowing what the past water tests results were.
Often, having water tests results will confirm what is observed by a surface analysis, and sometimes the available water tests results will contradict what is seen by surface analysis. It is nice when both (analysis) confirm the same plaster issue.
With this particular pool, I will be looking at the surface to see if etching has occurred, or if there is a calcium scale on the surface, or if a color pigment has been bleached. I will sand the surface and observe the color differences underneath. I may have to take (remove) a chunk of plaster and analyze it (in a lab) for its' calcium chloride content, water/cement ratio, cement/aggregate ratio, test the color pigment for colorfastness, and observe plaster the surface under magnification.
I plan to visit and inspect this pool within 7 days and will share what I see and learn in this thread.