Louai, the easiest way to check the pH is with a pH meter -- but perhaps you can do it the old-fashioned way that mozzarella is made, monitoring the curds by eye and nose as they continue to ripen, and when they begin to smell acidic (hard to describe, but quite distinct), checking a little bit of curd in hot water to see if it will stretch. If it does, you are right at the target pH. Sorry, I don't have a video ... but there might be one somewhere on the internet.