How to prove that a given colorless liquid is an acid

1. Using Litmus papers. Litmus paper will turn blue if solution is acidic or it will turn blue if it is basic ... If pH is below 7 or reddish then the given solution is acidic and if pH is higher than 7 or bluish then the solution is basic.

by using litmus papaer it will turn into red if solutions is acidic

and into blue if it's basic

by using Magnesium ribbon. Magnesium do not react with bases so hydrogen gas will produce when Magnesium is dipped in an acidic solution