How could you verify that an unlabeled soft drink is diet soda rather than soda sweetened with fructose?

To verify that the unlabelled soft drink is diet soda rather than soda sweetened with fructose, we'll do a test of the presence of sugar in the drink. Normally a diet soda contains aspartame, an artificial sweetener derived from proteins. A reducing sugar test will reveal whether the drink contains sugar or not. Absence of sugar will indicate presence of artificial sweeteners. The Fehling's solution and Tollens' reagent are two chemical solutions that test the presence of sugar in the drink. The Fehling's solution gives off a red precipitate in presence of sugar and Tollens' solution gives off a silver precipitate in presence of sugar.