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

File one of the bars so you have iron dust place it on a piece of paper and the magnetic lines will show in the dust if the magnetic one is under the paper and it will clump to the nonmagnetic iron if you filed the magnetic block or do nothing if you filed the nonmagnetic block. You could also heat up one block with a candle and see if it loses magnetic strength. If it doesn't you are heating the nonmagnetic block. Using the string you could hold them apart  to make it more sensitive to use less heat.