How Would We Know If Intelligent Life Existed on Earth Before Humans?...
[livescience.com]
Rock layer evidence, Fossil record, Dna...the usual lines of evidence that are the foundation of evolutionary model.
I don't think there would have been time for one to evolve before ours. And they would have left traces — possibly not obvious ones, but we'd have found them.
What the article is saying, I think, is what would those traces be & how would we spot them. They may be scant & rare, but knowing what to look for is key, IF there is anything to find.