You have two identical looking metal bars. One of them is a magnet, with North and South ends. The other is simply an iron bar (which is attracted by magnets).
How can you tell which is which, using only these two bars?
Lay one bar down. Then take the other bar and place the end of the other bar in the middle of the first bar. If the bars attract, the second bar is the magnet. If they don't attract, the first bar is the magnet.