Use a pH meter, or if not available, then universal indicator.
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.