Use pH paper to find the pH of the solution by dipping it into the solution and comparing the color the pH paper becomes to the pH paper chart that comes with the pH paper.
Multiply the pH of the solution by -1. For example, if the solution's pH was 5, you would get -5.
Raise 10 to the power of the result from step one to calculate the concentration of hydrogen ions. In this example, the concentration of hydrogen ions would be 10^-5, or 0.00001.
Divide 10^-14 by the result from step 2 to calculate the concentration of hydroxide ions. Finishing this example, you would divide 10^-14 by 10^-5 to find the concentration of hydroxide ions equals 10^-9.