Apparently The Portable Bernard Shaw (1977) included the quote, but did not source it.
This person claims:
I asked if he had heard George Bernard Shaw's quip, "The power of accurate observation is commonly called cynicism by those who have not got it." This brought a smile to his face, and he later asked if I could provide a citation. Unfortunately, I could not - it was just something I had heard and remembered - but today it only takes a quick search of Google.com to find many sources for Shaw's remark.
I wonder how he searched for it?
About the only time I could see Shaw using the word cynicism was in:
"The glimpses I have had of the seamy side of life during the last few months have made me cynical; but I should not have brought my cynicism here--least of all into your presence..."
~from Arms and the Man, 1894
by George Bernard Shaw