How can I avoid getting swindled? How can I detect polyphosphates in fish fillets?
Put the fish under a blacklight and dab a bit of 4',6-diamidino-2-phenylindole stain on it. If it changes glow from blue to green, its been treated with polyphosphate.

You can purchase DAPI stain here: http://www.sigmaaldrich.com/catalog/product/sigma/d9542?lang=en&amp;region=US

I would suggest cutting a piece of the fish that you will not be eating, dabbing the stain on that piece and tossing it in the bin after.

Or lend it to your chemist cousin :)