6) Watch their eyes. I can't give you any algorithms here but intelligent people have faster-moving eyes; average peoples' eyes have this slow "wrinkling" quality. This may be part of ..
7) with intelligent people you rarely need to finish an explanation or even a sentence. Not only are they tracking what you're saying, they're thinking ahead and following the ramifications of what they've already heard and reach the conclusion ahead of your explanation. Working with people like this is a joy.
I use the latter in a most manipulative fashion; if my listener/reader reaches the endpoint of the reasoning on his own then it's more convincing than if I spoon-feed it to him. His acceptance of the idea is coupled with his enjoyment of his own intelligence.
Good article. SO much nicer than the obvious articles on here and the eye-glazing software simplicity.