The physical size of the power supply and the area allotted for it is the
first clue. Next is how it mounts. Finally, the connectors and cable lengths
to the motherboard. Or, you might be able to just ask HP. There is also a
"standard" as to what voltages appear at what pins on the power
supply/motherboard connectors/plugs.

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ups.com...
How do you know if it's proprietary?