How do I test an xbox 360 slim power brick?
There is conflicting information online, but the gist is that the XBox 360 PSU has a power enable pin (Pin 7). The PSU will only provide 5v power, which is the XBox 360 standby power source, until the power enable pin is activated by the Xbox (When you go to turn the Xbox "on"). Until then, 12v is disabled.

The conflicting information is what you need to do with the Power Enable pin. Some say you need to tie it to ground, some say tie it to 3.3v, and some say it can be tied directly to the 5v standby source (I would use a resistor none the less). Since it is a proprietary power supply, everything we know about it is educated guesses about how it works by people tinkering and messing around with the Xbox. Considering the high amperage, please be careful, you could kill yourself if you do the wrong thing.

Search for Xbox 360 "power enable" pin on google for more information, at your own risk.