Assuming a human-like entity has the ability to visit a planet that is like (or is, if that makes this easier) Earth during two time periods:
- A time period where the planet's magnetic field is stable and Magnetic North is at Geographic North, and Magnetic South is at Geographic South;
- A time period where the planet's magnetic field is stable and Magnetic North is at Geographic South, and Magnetic South is at Geographic North.
Assuming also that
- there aren't any digital or written physical records from a civilization detailing the magnetic field shift as it was happening;
- the human-like entity lacks a magnetic compass to measure the change, and they don't have the ability to single-handedly check the sea floor for magnetic field stripes, at least until they have evidence that the magnetic field has already flipped.