Put on personal protection equipments like safety goggles, lab coat and rubber gloves in case you are dealing with hazardous substance.
Prepare a solution of the substance using proper solvents like water or alcohol or other organic solvents.
Check the pH of the solution using a universal pH indicator strip by just dipping the strip in it
Compare the color change on the strip with the color comparison charts available with the universal pH indicator strip. Ensure the pH value lies between 0 and 7 for the substance to be acidic.