Your pool water needs to be tested on a regular basis. When testing domestic swimming pool water, the most common way of testing is to use a comparator test kit which works by comparing the colours of a known concentration to a sample.
Each individual test kit is different but the principal is the same.
The ideal range for chlorine is between 1.0 – 3.0 ppm and PH between 7.2 -7.6, this is usually around mid-way on the colour charts and the test should show an orange colour for ideal PH and a mid-pink colour for ideal chlorine.
If the levels are out then small chemical adjustments are required and another test within 18 hours is recommended, when adding chemicals add little and often and re test regularly, this will keep a stable level rather than an erratic variation.
More advanced testing, generally used for commercial swimming pools can be carried out using a photometer which works by passing a beam of light through the water sample and providing an accurate result on a digital display. This form of testing eliminates variations in peoples vision and provides a much more accurate result which is required for commercial operations.