The best way to measure free and combined chlorine is with a test kit that does so such as the one Ben sells. But in a pinch, I sometimes will use this crude and somewhat inaccurate method with test kits that test total chlorine only. After adding the drops and mixing, immediately note what the color and reading is. This is close to your free chlorine level. After a few minutes, note the color and level again. This is your total chlorine level. Combined chlorine is then the total - free. If the color does not change, you probably do not have any more combined chlorine. Again, this is not very accurate and will miss small amounts of CC. Long term you might want to order Ben's kit.