Detecting Kombucha that's not fit to drink is pretty easy, but let's discuss some of the ways we can tell. The scoby will have mold spores on it. This mold will be black or green, and unlike the brown that may develop naturally as part of the old bacteria growing on a healthy scoby. Harmful mold will look like the mold that grows on bread. Do not try to salvage the scoby.
If you keep your Kombucha at a pH level between 2.5 and 4.6 there is little risk of contamination from foreign cultures. Acidic conditions are favorable for the growth of the Kombucha culture, and inhibit the growth of molds and bacteria. A pH of less than 2.5 makes the drink too acidic for ordinary drinking, while a pH greater than 4.6 increases the risk of contamination.
Another sign of Kombucha that should be discarded is any unpleasant aromas. If the scent is sulphurous, it is a natural by product of fermentation, but if the taste is also very vinegary or "off," the batch and scoby should be discarded.
Another contamination risk comes from insects. If the Kombucha is kept constantly and tightly covered, this kind of contamination should not be a problem. If the cover if left off, or if the covering is so porous that small insects can invade the space inside, and there is visible evidence of insects, the Kombucha should not be used. Insects can carry foreign bacteria into the culture, so discard the tea and the scoby.
With sensible, attentive, and clean habits, your Kombucha brewing can be on a continuous cycle for many years, giving you the satisfaction of caring for your own health, and providing you with an economical and delicious alternative to the usual beverages.