How do I know that fermentation is starting ?Updated a year ago

If you use a glass jar, you can monitor the fermentation. From the third day onwards, a small light layer should have formed on the surface, this is your new scoby. You should also see some yeast strands (brown strands).
You can also do a taste test every day. If your Kombucha becomes more acidic, this indicates that the microbes are working properly.