If it is indicated that a solution of a certain substance is alkaline, then how to prove this experimentally?

Experimentally, it is possible to prove that the presented solution is alkali in several ways. The first option is to make a chemical reaction with an acid or acidic oxide, then it can be argued that this is an alkali. The second way to determine the pH of a medium is to use indicators. Lackmus, phenolphthalein and methyl orange. If the litmus is dipped into an alkaline medium, it will turn blue, if you drop methyl orange, the solution will turn yellow, and if phenolphthalein is dropped into the alkali solution, the color of the solution will change to raspberry.