Identification of polyphosphate crystals

Hello everyone,

I would like to know whether there is a way to verify that the crystals in my anti-scale filter are actually polyphosphates. I am asking because they do not seem to be dissolving as I would expect.

I have had them for almost three years (or possibly longer). The crystals appear to have decreased in size, but strangely the overall level in the container has not gone down. This made me wonder whether there is a way to confirm that they are indeed what they are claimed to be.

I also considered performing a limescale test before and after the filter, but I am not sure how to set it up properly, aside from a simple empirical visual test by letting the water evaporate on a glass slide.

Thank you in advance for any suggestions.