> Assuming that the possibility of finding extraterrestrial artifacts on other
> bodies in our solar system isn't a lunatic notion, what sort of things
> _should_ we be looking for?
I don't think a civilisation capable of interstellar travel is going to engage in building megalithic structures on planetary surfaces. As far as we know interstellar travel requires micro craft built with nanotechnological means. Shortly after arrival it autoamplifies into a vast machine ecology living in space as their native habitat. There is no point for them to descend down the gravitational gradient for purposes other than large-scale mining (leading eventually to planetary deconstruction).
It is impossible to destroy such a supercivilisation by anything other than a grand cataclysm (say, a GRB event occuring almost in the system), which will certaily leave tell-tale traces everywhere.
If you're looking for alien artifacts in our Solar System you won't find any unless