I know most people have heard the usual tricks. Eliminate ahh's and umm's. Stop and ask questions. Repeat back what the other person said. Take pauses. Allude to facts and figures. Maintain relaxed and attentive body language. Make soft statements, rather than stating absolutes ("I suggest, I believe, it appears").
Based on my observations, I've noticed that the people I've judged to be "smart" (both emotionally intellectually) talk less. The crazy thing is that I never notice it.
There are several potential reasons why brevity can seem queen:
(A) They orchestrate conversations. They don't do anything snobby, like name-dropping. Instead they disarm and elevate their conversation partner. They listen, learn, and dig deep to try to empathize with the other's point of view. They nudge with nuance--with careful questions and timing. They manage to withhold any polarizing opinions and statements until they can find common ground.
(B) By reserving their words, every word that is said is received with more gravitas.
(C) Maybe because they talk less, I know less about them, and thus I form a more positive mental image about the person.
(D) Maybe they're just judging me silently.
(E) Maybe I have a sixth-sense for sociopaths.
Can anyone accurately judge smart-ness? I think not. Too often we conflate the seductive mastery of communication with being smart. Everyone communicates differently depending on how and where they've lived most of their lives. And it's likely that everyone has something they're smart at, even if it happens to be something you don't find very compelling.