That, and the real smoking gun: landfillite.

Some significant fraction of landfills are going to fossilize into a pretty distinctive sedimentary rock. I don't know what it would look like, but it would have, like, fossil Barbie dolls in it.

I don't think we'd have missed all of them if they existed. We could postulate an industrial civilization which just didn't make landfills? But we've only got ours to go on, and we really like burying trash. If our hypothetical forbearers were anything like us, we'd know about them.