How Would We Know If Intelligent Life Existed on Earth Before Humans?...
[livescience.com]
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.
Rock layer evidence, Fossil record, Dna...the usual lines of evidence that are the foundation of evolutionary model.