pH is defined as the negative logarithm(base 10) of the hydrogen ion concentration in moles per liter. Neutral solution has pH value 7. pH of acid solution is always less than 7 whereas that of alkaline solution is always more than 7.pH of pure water is 7. pH indicator is a chemical that has different colors in different media. Litmus solution and litmus paper are commonly used as an indicator. For example, blue litmus turns red in acidic medium, the red litmus turns blue in alkaline medium.
This video explains how to determine the pH of a sample by using pH paper / universal indicator.