You will first need to dissolve the substance in water, as acidity relates to concentration of dissociated hydrogen ions in the form of hydroxonium
Having done that, testing with a pH metre will allow you to determine whether the product is acidic. pH values below 7 are acidic.
If you have no pH meter, test the solution with universal indicator paper. Orange or red denote acidity (whilst blue and violet denote alkalinity, and green indicates "neutral").
A "sour taste" is found with some, but not all acids, and it depends to an extent on concentration, so beware of using this as a test.