Because alien planets can also bear structures reminiscent of those you could find on Earth, as well as similar fauna and flora, I think what would really hone in the fact that is earth is artifacts depicting human life.
Statues, carvings, paintings, photos, or anything that features a human figure within it. Along with this, old newspapers, or anything that is time-specific to a date that people can relate to, like fragments of a 60s diner with debris or decor that we associate with it.
But to evoke feelings of nostalgia or emotion, bringing life and a story to the inanimate, and making them characters in themselves.