Answer: By using litmus paper.
Step-by-step explanation:
Litmus paper is an indicator which helps us to determine the solution as acidic or basic. These are of two types: red litmus paper and Blue litmus paper.
Red litmus paper is resistant to Acidic solution but it changes its color to blue when dipped in basic solution.
Blue litmus paper is resistant to Basic solution but it changes its color to red when dipped in acidic solution.
Hence, we can use litmus paper to test whether the unknown solution is acidic or basic.