A water sample is suspected to be contaminated by PO4 ions. How could you know if the sample is really contaminated?

To 2 cm³ of the water sample, add a few drops of silver nitrate solution. If the water is contaminated with PO₄³⁻ ions, a yellow precipitate is formed which is re-dissolved in an excess of nitric acid.

see https://en.wikipedia.org/wiki/Phosphate_test