Using a multimeter or continuity tester check to see if the pins are shorted

There are five pins in the plug end. The central one is used for signaling but the other four are used to carry power to the mac. Because the mag safe adapter is designed to be able to used in any orientation the outer pins (1 and 5) are equivalent and should show as connected when using the multimeter.

The same should be true for the middle pins (2 and 4).

If however any of the other pins appear to be connected, you have a short. In this case pins 1 and 2 are shorted.