Some people do guess at it. I believe you need a test kit. They sell a strip test kit and a chromatography test kit. I think the later is a much better solution.
One reason to test is when the malic acid, which they convert to lactic acid, is all gone, they sometimes can start munching on other things and it can create problems. It really is a good idea to stop the MLB with sulfites when the malic acid is gone or near gone.
