If humans were to die off today - what evidence of human civilization might remain a million years from now?

There would be little surface evidence of civilization. However, a geologic layer would exist that would contain plastic and other non-biodegradable substances. Biodiversity below that layer would be greater than that above the layer, indicating a mass extinction event that is in the process of occurring at the moment. Ice cores would show a significant increase in atmospheric CO2, metals, and other substances that spiked and did not persist in the geologic record. There would be multiple lines of evidence that something extraordinary had occurred in a very short span of geologic time.