If you lived on the far side of the Moon, how could you infer the existence of Earth?
A body tide seismometer on the lunar far side would pick up both the solar tide, and the 20 inch body distortion produced by the earth.
While "tidally locked", the moon is not in a perfectly circular orbit, and also wobbles a bit; libration. Your seismometer should pick up both effects.

Watching the parallax of mars cycle every 28 days, as suggested in comments above, might be a simpler way to go.