As I mentioned in my answer to this question: http://worldbuilding.stackexchange.com/questions/46959/earth-nukes-mars/46964#46964, the give away for previous high tech civilizations would be radioactive isotopes.
If they had nuclear power or technology, they would have produced many non natural radioactive isotopes, as they decay predictably over millions or billions of years we would be able to measure the amounts of various isotopes, similar to how carbon dating works and tell that some non-natural nuclear reactions were happening long ago.
In terms of actual physical artifacts, things survive, that's how we know about dinosaurs and other ancient life via fossils. The obvious place to look for artifacts of an advanced civilization is in space, but micro-meteors and other space debris would actual cause wear, and most orbits decay or drift in the time frames your talking about, but say finding a lump of refined metals or other non natural material (polymers alloys etc.) would definitely be an indication of ancient advanced life.