You can easily determine the sugar content with a refractometer.
For the alcohol content, hydrometers and refractometers are recommended in the beer industry, but due to the very low alcohol content in kombucha and water kefir, the alcohol content cannot be determined very accurately.
In addition, kombucha and water kefir contain more particles and can therefore falsify the result. So for at home, I would advise you to trust your taste.