I figured someone would have said this by now, and it seems obvious to me, but I'm going to keep in mind the general principle that what seems obvious to me may not be obvious to others.

You said efficient ways to signal intelligence. Any signaling worth salt is going to have costs, and the magnitude of these costs may matter less than their direction. So one way to signal intelligence is to act awkwardly, make obscure references, etc.; in other words, look nerdy. You optimize for seeming smart at the cost of signaling poor social skills.

Some less costly ones that vary intensely by region, situation, personality of those around you, and lots and lots of things, with intended signal in parentheses:

• Talk very little. Bonus: reduces potential opportunities for accidentally saying stupid things. (People who speak only to convey information are smarter than people for whom talking is its own purpose.)
• Talk quickly.
• Quote famous people all the time. (He quotes people; therefore he is well-read; therefore he is intelligent.)
• In general, do things quickly. Eating, walking, reacting to fire alarms. (Smart people have less time for sitting around.)
• During conversations, make fun of beliefs that you mutually do not hold. Being clever about it is better, but I don't know how to learn cleverness. If you already have it, good. (He is part of my tribe and one of my allies. Therefore, because of the affect heuristic, he must be smart as well.)
• Learn a little bit of linguistics.
• Tutor people in things. (You have to be smart to teach other people things.)

It was not intentional that all of these related to conversation. Maybe that's not a coincidence and I've been unconsciously optimizing for seeming smart my entire life.