A careful examination of the fossil record and the distribution of species around the planet, would also reveal an era (in geological terms extremely brief) of unusually rapid long distance migrations of plants and animals; as well as the mass extinction). This is evidence independent of the direct discovery of fossil humans and human artifacts.
I suspect all these strands of evidence would be diagnostic of an intelligent species which was not wise enough to avoid global catastrophe.
Of course, as the ultimate technological fix, we might have downloaded our minds to machines or created artificial intelligences as our successors; so the aliens could get a first hand account of what happened.
One funny possibility is that we would leave enough evidence to suspect existence of our civilisation of this level, but not enough to establish it up to scientific certainty. We can imagine a bunch of rather gifted allien enthusiasts of Terrestial civilisation being derided by their scientific "establishment" :-)))
Intelligence detection is an interesting problem for science. Positive confirmation by single artifacts looks more feasible than falsification of the hypothesis by single observations. What would Popper say?
