You have been given three test tubes one of them contains distilled water and other contains the acidic and basic solution. If you have given only a red litmus paper how will you identify the content of each test tubes?

Take a drop of solution from each test tube on red litmus paper, the solution which turns red litmus blue will be basic in nature (for example NaOH)

Now add this basic solution to the other two test tube. The test tube containing distil water will become basic in nature(it will turn red litmus to blue in colour) and the test tube containing acid will react with the base to form the salt. (red litmus paper will not change colour).