Iron filings can be used to visualize the magnetic field lines of a magnet.
For example, from Wikipedia:
The magnetic field of a bar magnet revealed by iron filings on paper. A sheet of paper is laid on top of a bar magnet and iron filings are sprinkled on it. The needle shaped filings align with their long axis parallel to the magnetic field. They clump together in long strings, showing the direction of the magnetic field lines at each point.
Question: When looking at magnetic field lines formed by iron filings, how can we determine which magnetic pole is North and which is South?