The thing about buying a white powder: how does one know if it isn't cornflour?
Same problem with buying cocaine, I guess.
If you really want to know:
Dissolve about 15 mg of the substance to be examined in 2 mL
of water R or use 2 mL of the prescribed solution. Add 1 mL of
dilute ammonia R1. A white precipitate is formed that dissolves
on addition of 1 mL of ammonium chloride solution R. Add
1 mL of disodium hydrogen phosphate solution R. A white
crystalline precipitate is formed.
Probably not at the high street chemist. We used https://www.sigmaaldrich.com/GB/en/products/analytical-chemistry/analytical-reagents