I think the skill is mostly learned from experience with human interaction. That said, I think there is some talent involved as some people seem to pick up on the signals more quickly and more accurately. However most people with some experience can learn to become more effective people readers.
5w6 or 9w1 sp/so/sx, I think
As part of a process, I think accurate reading requires the ability to analyze one's unconscious with relative thoroughness.
How one accomplishes that is up in the air, i.e. "I need to withdraw" vs. "engage".
So there are varying levels of spontaneity and efficiency.
I've found that, with age, I slide into progressively deeper thought, perpetually spinning impressions together, mapping them around my mind.
How do these create my reality. How can I plan my thoughts around these while minimizing inefficiency.
Assuming it's a challenge for most to determine (from a single perspective) whether nature or nurture has more of a say in how one develops, understanding where the fine line between the two lies should be a goal for more withdrawing types.
How does one fit in the larger puzzle. Can you visualize the light at the end of the tunnel.