If they use radio telescopes or spaceships, we know they are intelligent.
If they don't, the issue will never come up, so it really doesn't matter whether they are intelligent or not.

In SETI, "intelligence" is defined as some activity that we will recognize as associated with intelligence and that we have the equipment capable of detecting. If the ETIs are invisible spirits who communicate by telepathy in some other dimension, we probably will never contact them.

The reason this "anthropomorphic" view dominates SETI is

1) We can't do SETI at all unless we make that concession, we call it
"only looking for our car keys under the lampost" because we can't find them at all in total darkness.

2) We have good reason to believe technologically oriented species will be curious about the universe and they will study it with equipment (like telescopes and spaceships) also suitable for SETI purposes. The only way we know the universe can be studied (or messages transmitted) over interstellar distances is by the generation and detection of electromagnetic radiation. There may be other ways, but EM radiation will already be familiar to astronomically-capable civilizations.

3) We can only communicate with technological species because only technology can generate any activities detectable by other technologies over interstellar distances. Maybe there is some form of inter-species telepathy we don't know about, but I wouldn't count on it.

Everyone concedes that some very interesting civilizations may indeed exist which don't meet these criteria, but they are not going to do us much good because we'll probably never find out each other anyway.

The important thing to remember is that SETI is something that we can do right now with what we already know. It is not magic or wishful thinking. We KNOW how to transmit and recieve messages over interstellar distances. We already have machines that can do this, this makes it a scientific enterprise and an engineering possibility, instead of mere philosophy.