Imagine a cuboid planet with a thick atmosphere orbiting a star similar to our Sun. The inhabitants are as intelligent as humans and recently discovered the universal laws of gravitation and motion which are familiar to us. How can they determine that the planet is a cuboid? The dimension is 1024 cubic kilometers and for some unknown reasons the cuboid planet is formed and has remained that way ever since.
I'm looking for primitive technology to prove the planet is a cuboid with the specified dimensions with smallest error margin possible.