Add a few drops of acidified potassium manganate(VII) solution to a solution containing an unknown agent.
If a reducing agent is present in the solution, the KMnO4 solution will turn from purple to colorless.
In the presence of a reducing agent, a redox reaction occurs, turning the Mn7+ ion in KMnO4 (which is purple) to Mn2+ ion (which is colorless or slightly pinkish).