Take the two bars, and put them together like a T, so that one bisects the other.

  • -------------------+
bar A ->|
  • -------------------+

    • --+

    | | | | | |

bar B ---------->|

| | | | | | | |

  • --+

If they stick together, then bar B is the magnet. If they don't, bar A is the magnet. (reasoning follows)

Bar magnets are "dead" in their centers (i.e., there is no magnetic force, since the two poles cancel out). So, if bar A is the magnet, then bar B won't stick to its center.

However, bar magnets are quite "alive" at their edges (i.e., the magnetic force is concentrated). So, if bar B is the magnet, then bar A will stick nicely to its end.

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