That Edgar Allen Poe, the reputed father of the mystery story, sure knew how to size up a guy. Here's a marvelous quote from his famous short story The Purloined Letter.
The speaker is an eight year old boy who is always winning at a game of marbles where you guess whether the person is holding an odd or even number of marbles in their closed hand. He figured out how his opponents would play based on their astuteness. When asked how he identified the other's intelligence he replied :
“'When I wish to find out how wise, or how stupid, or how good, or how wicked is any one, or what are his thoughts at the moment, I fashion the expression of my face, as accurately as possible, in accordance with the expression of his, and then wait to see what thoughts or sentiments arise in my mind or heart, as if to match or correspond with the expression.”