This is off the top of my head but, if needed, we can dig deeper into it.
To get this started maybe we can consider what I think SETI is trying to do.
First they have a wide range of samples of what are taken as non-designed signels (as noted above). So they are looking for something "different" in some way.
However, what differences count?
First an understanding of natural processes allows the exclusion of some signels.. The famous LGM signel of 1968 (IIRC) was quickly relegated to natural when an explanation of it was figured out (actually, IIRC the explanation already existed they just had to tie them together).
I know that one idea is to use a series of prime numbers as a strong indicator of intelligence since no one has figured out a natural process to produce that.
At this point those familiar with biology and evolution will recognize the contrast with ID.