I've often wondered the same thing!

A little while back two NASA scientists published a paper on how you would detect the existence of a previous advanced society on Earth. Apparently, over a long time scale it's actually fairly difficult.

Not at all my area, but it doesn't seem like we can actually answer that question you posed. It's fascinating. Carl Sagan's COSMOS might be the first time I'd ever heard about Global Warming, and he explained how it was through the study of Venus that we really began to understand it.