The pH test can be done in different ways. Two common methods use either a pH meter or a pH comparator kit.
A pH meter is a piece of equipment which measures the concentration of hydrogen ions in a solution. The pH meter uses a probe called an electrode, and a dial with a pH scale. The electrode and the ion meter sample a solution, "calculates" the H+ concentration, and records it on the dial. These pH meters are very expensive if highly accurate readings are required.
A pH comparator kit is designed to calculate pH using indicator chemicals.
One type of pH comparator uses acid-base titrations (a titration is a method of measuring the amount of something in a solution; in this case, the amount of acid in rain). With titrations, an indicator chemical (e.g., phenolphthalein) is added drop-by-drop to the sample. The number of drops needed to change the color of the sample are used to calculate the pH of the rain.
Litmus paper uses indicator chemicals, which are combined with a special absorbent paper. When this paper comes into contact with anything acid, it will change to a certain color which, when compared to a chart of standard colors, indicates the pH. This litmus paper is made for both acids and bases and is inexpensive (The results will not be as accurate as using a pH meter, however).
The final type of comparator kit uses chemicals, which are added to the rain sample. The indicator will change the color of the sample, which is then compared with the appropriate standard color and pH.