If an advanced, intelligent species had once inhabited Mars, but went extinct for some reason, would we have any way to know it today?
Depending on how technologically advanced the civilization was some things will be visible.
Archaeological evidence of structures, tools or even machinery could tell us a lot.
The images below show some of the abandoned ancient machinery found on the planet Mars. Click on the images to enlarge in 1280 x 720 (HD).
Link to the panorama view MSL Curiosity SOL 1450-2 processed by Keith Laney: http://gigapan.com/gigapans/219339