@cstross

Certain "artifacts" can last a very, very long time. A long, very straight line, cutting through normal geography, is easily identified as a manmade road - in some places likely lasting basically forever until something like tectonics disrupts it; I'm thinking about the glacial scrape marks in the rocks in NY's central park that match up with others in Europe (forget where) that survived millions of years of continental drift.

I also expect a future civilization would detect us by plastics - some of which never degrade naturally, until something evolves to degrade them, which may be never. That is, if the radioactives didn't already give us away.

On a more human scale - carved stone and glass or ceramics could easily go for millions of years if well protected - burial works.