For quantification you may need to lyse the cells. You can start taking a look here:
Direct quantification of inorganic polyphosphate in microbial cells using 4'-6-diamidino-2-phenylindole (DAPI)
http://www.ncbi.nlm.nih.gov/pubmed/21875055
For the qualitative estimation you maybe could use microscopy with a filter that allows wavelengths over 500 nm but no below, as DAPI-DNA complexes fluoresce at around 450 nm (blue) and DAPI-PolyP at 525-550 nm (green). You can take a look here, although it is focused on eukaryotic cells and polyP containing organelles it might be suited for bacteria
New insights into the in situ microscopic visualization and quantification of inorganic polyphosphate stores by 4’,6-diamidino-2-phenylindole (DAPI)-staining