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If it weights 5KG+ (12pounds), likely it is not a switching power supply as it has a bulky transformer (I can't imagine a server PSU not being a switching one). Other than that laptops, phone chargers, etc. would work off DC. Most of them use a small transformer and high frequency (above hearing range)to drop the voltage.

The transition is something like: 90-240V AC (50-60Hz) -> DC (1.414*AC) -> AC high frequency ->(transformer) AC low voltage -> DC 12V

Very cheap electronics that use capacitor+resistor voltage drop need AC.

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To answer the question directly, use a small fuse 1-3A and run the PSU idle. Like advised by sibling, measuring the resistance would be sufficient as well.