Can you solve the MAGNET RIDDLE? ft YouTube CEO Susan Wojcicki

7:20 just use the file to scratch the surface of one of the bars. If they don´t stic to the surface, then the other bar is the magnet, so aproach them to the magnet to prove it is. 

You can also heat up one of the bars (if you had a blowtorch) as it will lose the magnetic field (maybe forever) This is due to the different tempering of steel, I don´t know if it was "martensita" (spanish name) or "austenita". It could be "perlita" or "cementita" as well, can´t remember the diagram of the tempering, it was too complex.