So my ISFJ mom and I were talking; she mentioned how someone had commented to her that my little bro (INFP) was sitting alone in the field at his school; should they be worried? She replied, "no"... anyway she was relating the story to me, my response was, "of course not, he's just daydreaming".
She looked at me blankly, and said, "no. i've done stuff like that too. Sometimes I just like to sit and listen to teh birds and look at the beautiful grass and enjoy the nice clouds..."
And I was like, "No. He was daydreaming".
And she responded, "No. You're brother's a deep thinker" We bickered; I knew I was right.
I asked her how often she daydreamed. She replied "never". She asked me, I said "several times a day, often for long periods".
When I told my bro about this later, he laughed until he realized I was serious when she thought he never daydreamed. His response? "What does she think deep thinkers do?!"
So we asked my ESFJ sister; she daydreams "maybe once a month". My bro talked to three of his friends; two thought he was crazy for "having conversations in his head" and one could totally relate.
I think we've stumbled on the jackpot, guys. Just ask someone if they daydream, and if so, how often. There yah go. That's the divider!
(Although a very balanced S/N might not give a definitive answer, I suppose.)