The other day I went to the Ardenwood Historic Farm, which isn't far from our house. It was peak season there, with a corn maze and pumpkin patch and lots of great animals at the farm. One of the key attractions was a pair of pigs - huge creatures, which we were told had only been born this spring. The friendly farm volunteer asked us if the pigs looked intelligent. Some folks, mostly kids, said no. Some sagely nodded their heads, having been told previously that pigs were quite intelligent. Then the volunteer explained to us that one of the big signs of porcine intelligence was this: they might spend a lot of time lolling around and digging their heads and bodies in muck, but they keep the location where they defecate constant, separated from the location where they sleep, and the location where they eat. Other animals at the farm don't do this.
It got me to thinking. What are the cues we use to measure intelligence?
This is a question that is quite relevant to science fiction, because in the science fictional arena, you quite often find situations where one alien or another is casting aspersions on another species' intelligence (or possibly admiring it). But it's not just for alien scenarios. When we think to portray a character to our readers - just generally - we need to make sure their impressions of that character's intelligence agree with our own, and it's useful to think about what kind of cues to use.
We might be able to run a dog, or an alien, through a maze, but that's a bit less plausible for humans.
A lot of the way we assess intelligence comes down to behavior. A creature or person who is keenly attentive to his surroundings, and purposeful in his movements, can easily come across as more intelligent than one who is not. Restraint and good manners are often taken as a sign of intelligence. Someone can show intelligence in the way she organizes her belongings or the space she occupies. In a conversation between speaking entities (not necessarily human!) a shorter lag time before beginning a response can indicate intelligence. People who speak faster can often be seen as more intelligent - and I've even seen this measure used (naively) to cast aspersions on speakers of dialects that have a slower rate of speech. Another sign of intelligence in speech is lack of redundancy - either the lack of reiteration in one's own speech, or the ability to build on another person's contribution to a conversation rather than simply recasting it.
These measures can be useful - and the expectations associated with them can be used to the writer's advantage in description and dialog. However, they are heavily influenced by culture and context, and should be considered highly fallible.
A person isn't unintelligent if she speaks with a slow dialect. Leaving a pause before you respond to what someone says could be interpreted as a lack of alacrity - or as thoughtful pondering of a response. Good manners are just that - good manners. And we know that all sorts of people get crazy once in a while!
Misunderstandings in this arena can provide great opportunities for stories. In the story I'm currently working on, the group of aliens has an unusual criterion for "higher intelligence" that humans don't happen to share. The pigs at Ardenwood farm are a great example of how a behavior that might seem senseless to us - digging around in mud with one's nose - can make perfect sense to another species and have nothing at all to do with that species' level of intelligence.
It's something to think about.
After reading through the links that Mike Flynn volunteered in my comments section below, I heartily recommend his taxonomy of aliens, which can be found here.