[kortex]

There are a lot of things which would likely not break down oven millions of years, provided they were largely protected from the element. A lot of glasses, ceramics. Tungsten carbide (pretty much any super hard machine tool, as long as it wasn't subducted). The isotopic distribution of nuclear waste would be a big clue.

And anything we've stuck on the moon.

Not to mention, all of the LACK of things. Giant holes from mining operations. All the straight cuts we've made through mountains. The Panama Canal.