The best way to pull a sample is with a straw. Don't drink directly from the straw, as backwash may contaminate the tea. Also, do not dip the test strip into the brewing vessel. Instead, dip the straw about halfway into the tea, cover the end with your finger, pull the straw out and drink the liquid inside or put that liquid on the test strip. If the kombucha tastes very sweet, it probably needs more time for the culture to consume the sugar. A pH of three tells you that the brewing cycle is complete and the tea is at the correct point to drink. Of course, this can vary a bit to suit your needs and taste. If this final pH is too high, then either the tea will need a few more days to complete the brewing cycle, or it should be chucked.
Source : http://thesouthern.com/lifestyles/magazine/food/beer/cheers-to-beer-step-by-step-guide-to-making-your/article_9e743d82-d626-570c-933e-0d3cfaa5a741.html