While I certainly see value in the idea of individual "letter" differentiations such as raised in these great threads....

To me, it is more natural/effective to guesstimate people's personality type in terms of what Keirsey would call the "temperaments": NTs(Rationals), NFs(Idealists), SPs(Artisans) and SJs(Guardians) -- rather than trying to first sense I vs. E, then S vs. N, etc ... one by one. (The one-by-one approach can help in sorting out the remaining characteristics of an already sensed temperament basis, however)

For instance, and quickly... (in terms of often supervening if not salient behavioral patterns, as I tend to see them)

NF - Idealists
  • imaginative and enthusiastic in behavior centered on their beliefs

NT - Rationals
  • abstract and logical in behavior centered on their pursuits

SP - Artisans
  • adaptive and reactive in behavior responding to the immediate

SJ - Guardians
  • attentive and practiced in behavior responding to the established

By behavioral, I mean you can just listen to what people say and how they say it -- "between the lines", at times, outright at others. Notice what they appeal to, what they make reference to, etc. Whether they sooner to tell you something, or just suggest it, etc.

Anyhow, once I begin to guesstimate one of these four, I then use the sorts of things mentioned in the other threads to differentiate the remaining "letters".

What are some ways you've learned to quickly sense NT, NF, SJ or SP personalities?