Usually when I type someone, it's because something they said or did made one or more of the letters obvious. When I talk to people, I'll often get vibes such as:
"That was such an SFJ thing to say."
"They get it, must be NT."
"Why can't she just decide already? Must be a P."
Then I go from there and fill in the other letters with what makes sense.
I can't turn it off. It's hard for me NOT to type people. Probably not a good sign.