Checking the pH for an accurate reading of acidity is a good way to ensure other pathogens do not grow in your tea. If the kombucha starts to smell cheesy or unpleasant in any way, this is a good indicator that pathogens invaded your brew. You will then need to discard your brew, and start over again with new tea. If there is mold on the SCOBY, both will need to be discarded, and you will need a new SCOBY as well. A SCOBY can last a very long time, but it will expire. If the SCOBY turns black or otherwise has many dark spots, that is a sign that it's time to get a new SCOBY. If it develops green or black mold, it has been invaded by pathogens. Either way, you must throw away the SCOBY and start over. (3)