There is no real defining line for "intelligent." Heuristically, we have defined intelligent as:
- Human beings are intelligent
- Other animals are not (with some species that are questionable like Chimpanzees and Dolphins)
So pretty much, we define "intelligent" as "able to think in the way humans can but animals cannot." It's not a very satisfying definition, certainly not sufficient for exploring alien species.
We, of course, have IQ tests. Pattern matching tests like Raven's Progressive Matrices have been used to measure "intelligence" for quite some time. They do, of course, leave much to be desired, which is why few people put much stock into IQ tests.
However, I'm not sure of "intelligent" life is really what we're looking for in alien species. There's another class of words which I find more applicable: sentient, conscious, self-aware. If I was out looking for alien life, those are the adjectives I'd be looking for.
One metric you may find very interesting is the Phi function from Integrated Information Theory (IIT). IIT posits that consciousness can be measured in terms of how much information a system can process which cannot be explained simply by the sum of its parts. For example, the Phi of a block of RAM is basically 0, because RAM can't do any processing on its own. The Phi of a CPU is low, because it's heavily limited by its lack of space to store temporary values, but it can indeed do things which can't be explained simply as a sum of ALUs, caches, interrupt handlers, etc. Put them together, and the Phi of a computer, which is greater than 0 because it can do lots of things that can only be explained by the entire system. Break it up into its components, and it can't do much at all.
In IIT, a computer has some modicum of consciousness, because the metric IIT uses to define consciousness is one which is nonzero for a computer. However, the phi value for living animals is vastly higher than that of a computer, suggesting that animals are more conscious than computers. The Phi value for humans is presumed to be even higher than that of other animals, giving rise to the theory that we are the only conscious animal (IIT would argue that all animals are conscious, just to a lesser degree).
What's interesting about IIT is that that metric, Phi, is theoretically calculable for a system. Thus, if you came across an alien individual, you could theoretically determine a Phi for them, and use that to determine if your alien is "conscious" or not, by whatever line in the sand you want to separate "conscious" from "not conscious."