Some things are more of a genuine litmus test than others ;)
Most people are not that good at this either, but this one is easy.
As a simple sanity check, 6.6 is so much closer to 7 that the alkaline volume will dominate.
You can also take the 0.4 that the 6.6 deviates from 7 by, subtract it from the 9.1 and get 8.7.
Also, in electrochemical measurement, pH 7 = 0 millivolts.