What are two ways to measure the pH of a solution?
Measure H+ concentration or measure OH- concentration
Since pH is basically the number of H+ ions in a solution, that's what you're trying to find. (it's actually -log of H+, which is irrelevant since this is a conceptual question). You can do this with a variety of chemicals.
And pOH is always 14-pH. So you can also use the number of OH- ions to find pOH, which can find pH