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Trying not to be history. 12VDC 500ma power supply to 12VDC 300ma computer fan

I have a project in mind which would utilize a 12VDC 300ma computer fan. I would like to use an old peripheral wall plug style power supply. I have one rated at 12V 500ma which I think would work fine. 

Problem: How do I determine +/- if I cut the male plug off of the power supply without shorting out my apartment building or spending the weekend in intensive care? The cord is marked along one lead with a gray "dash". 

splice the ends of the wall wart twist connect the wires with the fan

if the fan is moving in other direction than intended then change the wires connected.

add heat shrink tubing or use electric tape to seal off any exposed wires

Perfect! Thanks for the info!!