Phosphate in water samples can be measured using several different methods, but ready-to-measure kits are one of the fastest.
Ready-to-measure kits are typically based on colorimetry methods. This means that the analysis is based on the change of color of a substance present in the measuring strip when exposed to phosphate. The more concentrated the phosphate is in the water, the darker the color will be in the strip.
Figure 1: Phosphate strips example.