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

The easiest way to determine the pH of a solution medium is to use three well-known indicators. Lackmus, phenolphthalein and methyl orange. If litmus paper is dipped into an alkaline medium, it will turn blue, if you drop methyl orange into the solution, the solution will turn yellow, and if you drop phenolphthalein into the alkali solution, the color of the solution will change to raspberry.

Another way to prove an alkaline environment in a solution is to react with an acid or acidic oxide, if the interaction occurs and salt precipitates, then the solution is exactly alkali. Since such a reaction will not occur with bases or alkalis.