The quote is "How can I tell what I think till I see what I say?"
It comes from E.M. Forster's "Aspects of the Novel" (1927).
Here is the quote in context:
'Another distinguished critic has agreed with Gide--that old lady in
the anecdote who was accused by her niece of being illogical. For some
time she could not be brought to understand what logic was, and when
she grasped its true nature she was not so much angry as contemptuous.
"Logic! Good gracious! What rubbish!" she exclaimed. "How can I tell
what I think till I see what I say?" Her nieces, educated young women,
thought that she was passée; she was really more up-to-date than they
source: page 101, Aspects of the Novel by E. M. Forster, Harvest
Books, 1956 edition, read using "Search Inside" on Amazon.com:
"know what i think till i"
"How can I tell what I think " forster aspects
amazon.com, "aspects of the novel" forster
I hope this helps.