*Very* rarely, an outlet may have a manufacturing defect where the hot and neutral is shorted. (As in I have only seen this happen once.)
Then other than that, turn off the main breaker, disconnect ALL the wires going to the circuit and use an ohm meter or continuity tester to test the wires.
Remove the light bulb first, then there should be NO continuity from any wire to any other wire.
Hot to neutral.
Hot to ground.
Neutral to ground.
When you see from the breaker panel that there is a short between two wires, you can then disconnect the wires further down the line and test from there.
Sometimes a metal clamp may cut into a wire causing a short. Or there my be a bare wire pointing out and touching a grounded electrical box. Or there may be a staple *through* the wire shorting it. Or when pulling wires with a lot of force through a hole in metal, this can strip the wire and cause a short to ground. Etc.
How to use a continuity tester...
http://www.dinosaurelectronics.com/Test_Fuse.htm