How do you recognize an acid?

Litmus paper is the cheapest way to go. Acids turn blue litmus paper to red. Some acids are recognized in other ways. Nitric acid gives off reddish-orange vapors on contact with humid air, hydrochloric acid has greenish vapors. Hydrobromic acid gives off orange (not reddish) fumes.

You can also do another chemical test to determine if the substance is acidic or not. This process uses a pH indicator, a substance that changes color depending on pH. This can be the same litmus paper, but you can also use red cabbage juice. You can also use a pH meter, which will give you a pH reading. Anything that dissolves in water that causes the pH of the solution to drop is acid.