I mentioned in my last comment to Dedwarmo
Is There Life in Outer Space? (Part 1) that if he found the words, "Steven Loves Priscilla" scrawled in the sand at the beach, he wouldn't scratch his head and wonder if it was brought about by the receding tide. Likewise, one would not wonder if wind and rain created the heads on Mount Rushmore, even if he had never heard the name Gutzon Borglum or seen another carver's work.
What do the letters on the beach and the heads on Mount Rushmore have in common? The answer is information.
That is what SETI scientists are looking for. And information has two components: specificity and complexity.
Something is specified if the arrangement has meaning. The word "the" is specified, but not very complex. If you randomly pulled letters out of a hat and got the word "the," you wouldn't assume the game was rigged, nor if scientists received the word "the" from space would they assume it was caused by intelligence.
Random noise is quite complex, but it doesn't mean anything. One way of defining complexity is that which has a low probability of occurring randomly. For instance, the following 30 letters have an extremely low probability of occurring in any random arrangement of 30 letters: ldjhoirojhksdghhlnjdlbhkcjmnjf. They are complex, but not specified, since they have no meaning.
Now if scientists heard, "Four score and seven years ago, our fathers brought forth, upon this continent, a new nation..." they would immediately assume the source was intelligent because the data is both specified and complex. That's information and information always comes from intelligence.