You would need to look for trash heaps and landfills. They would be mostly composed of organic household waste that would decay, but they would contain large quantities of non-organic materials that have no business being where they are. As a modern example consider expired solar panels. If I remember correctly they are composed of gallium doped silicon wafers and copper wiring supported by an aluminum or plastic frame and shielded from the weather by glass. While an ancient civilization may not have had solar cells they would likely have unusual concentrations of materials such as this. Also consider potential absences of material where it should be. All around the world there are abandoned mine shafts but I have yet to hear about such a thing that could not be traced to human endeavor.