Besides all the moral relativism that permeates post-modernist philosophy shown on this topic, I will risk writing a more traditional, dialectics based answer.
What is intelligence?
Well, lets do a dialectical analysis of evolution of life in our planet.
A mineral/chemical world is one where all the observable phenomena is the result of physical or chemical interaction between substances. Energy tends to fall and the entropy of the universe tends to grow. Biological life starts out of this chemical world because it is a superior form of chemistry. We might say that biological life makes a better use of chemical energy, hitchhiking the energy and the general increase in entropy of the universe towards its own perpetuation. Once life sets in, competition starts and results in a trait: to preserve itself, life evolves from more concrete, hardwired logic, towards abstract reasoning.
If two lifeforms are intermingled in a pattern of predation, or other form of competition, both will evolve due to limited resources. From chemical warfare of single cells, to group defense of multicellular organisms. As soon as a limit is reached a very "creative" adaptation sets in.
For example, when a cheetah hunts a thompson's gazelle, both evolve. Soon we reach a point where evolution is not possible anymore, and the genetic pool stabilizes. If a cheetah was to evolve indefinitely (in a specific niche that does not change) it would soon reach a point where adding muscle mass would decrease its speed. In other words, certain traits (in this case, speed) are limited by the composition (the chemical makeup) of normal life forms. If a cheetah was going to be faster, he would need to be made from much different chemicals, like iron bones, or something like that, which is much more costly to be used. The result is that this "impasse" is solved dialectically by a lateral operation. In the case of the primates, that lateral evolution was the increase of intelligence. Primates are not the utmost predators of nature. They are not stronger than lions, nor faster than cheetahs, et cetera. But they use their intelligence to hunt in groups using lances and other instruments.
This means that as soon as a limit is reached at a level, a certain new trait differentiates one group of species from others. For one, the beasts are still stronger physically than humans, but we reached a technological level that allows us to dominate all beasts without too much problem (and we've done that).
But why do life tries to perpetuate itself? Because if you have a primitive world where there are life forms that don't try to preserve their own existence, this non-self preserving life will be killed and replaced by any life form that developed a trait of self preservation. As soon as the ambient is totally filled with life, competition will set in, and self-preservation will become the utmost objective.
Even if we are talking about another planet, and our curiosity and romanticism wants to see something different (to allow us to dream with a universe where men did not dominate almost everything, and there is nothing in ordinary life out of our domain) this another planet is in the same universe as ours. It's composed of atoms just like ours, and those atoms make molecules. Provided that some differences might come from different makeovers of such planets (silicon vs carbon, etc.), those differences pale in comparison to basic similarities, like the need for energy, the competition, etc.
So, whats the purpose of intelligence?
Animals, plants and similar beings evolve by changing their own genome to adapt to circumstances. Provided a fixed habitat like I said in the early part of this text, animals would stagnate in ideal forms without reason to change. Our habitats are not fixed, but changing continuously. This means that animals must adapt out of natural selection, which is slow, and while, usually, able to save species from extinction, natural selection cannot save specific individuals from death out of inadequacy.
As soon as an animal is able to adapt the environment to his own needs, on a much larger scale than most animals, this single specific animal can save not only his species but himself from death out of changes in the ambient. The intelligence allows a much more flexible approach to competition with other animals. Instead of death of the weak and survival of the fittest, we get power over the ambient to change it as needed, and the more we evolve intellectually, the more we can adapt the ambient to our tastes.
So, regarding your question about how to detect intelligent/sentient life, we might answer the following:
- Sentience can be detected out of the animal capability to react
quickly and unambiguously to outside stimuli. A plant can react to
its external environment, but not quickly or unambiguously. A sentient
animal will have a basic reaction called fear. The need to save his
own life at any cost.
- Intelligence is a superior form of sentience. The animal at hand
will show capability to change its environment in a non-spontaneous way
in order to fit it to its own needs. We call that adaptation of the environment into human made forms "anthropized environment", or the geographical space unambiguously changed from natural to man made.
Life evolves intelligence to allow better adaptation and survival of the fittest. Intelligence allows changing the environment to something that suits the intelligent life-form better. This means that a intelligent/sentient life form will show a trait of environment manipulation that can range from simple tools and hunting tactics to terraforming and exploration of extreme energy forms.