Say human beings went extinct. How long would it be detectable to a visiting alien that intelligent beings once lived there? I'm not talking about infrastructure that disappears relatively quickly. For example: we knew an asteroid impacted earth 65mya long before we found the crater because of a sedimentary layer of ash that envelopes the planet. In the early 1900s we pumped tons of lead into the air, then during the later half, we dumped tons of nuclear waste into the air. How long would the geology show the telltale signs of a fire-controlling / nuclear capable species?