My quickest rule of thumb is whether they have a sense of humor or not. Bonus points if they speak English as a second language and still get the joke, (because I make jokes all the time). Beyond that, if they’re patient enough to listen to someone else’s speech (whatever it is) to the end before they start with a response, a rejoinder or a “Oh, that’s nothing, you should hear my story…” speech of their own, then that indicates a high level of “emotional intelligence”; I think that “there’s a person who knows how to have a conversation, at least.” (Sadly, I often find myself lacking in this regard; I’m working on it. I don’t suffer fools and blowhards gladly. Even worse, I can be a blowhard, too.)
Beyond that, though, a demonstrated and articulated ability to use logical reasoning and analysis – especially on the spur of the moment – is a sure tell of an active and facile brain at work. (But even someone who can do and demonstrate the reasoning, even if he or she can’t articulate well, still shows “intelligence”, even if the oral / verbal handicap is present.)
Then there’s always the ability to synthesize, to mix metaphors deliberately (or numbers and colors or other not-necessarily-related things) to add descriptive or analytical awareness, and the ability to analogize. Good analogies are really helpful to demonstrate a person’s capability to understand and break down a complex process – and to relate on a simpler plane to others who do not yet have that level of understanding – to instruct and illustrate.
Quick facility with numbers; figuring out a tip on a bill, for example, without having to break out a calculator, is another indication.
But not all forms of intelligence are equal, either, and they don’t always relate.
When I meet someone who can do all of the above, then I know that I’m in the presence of an intelligent person.