It should only
take a second to check for ground, you probably won't even
have to open the case.
Assume I'm dumb.
How?
OK, first find a connection that you know is logic ground. Things like
the ground pins of a parallel port, the ground pin of an RS232 port, the
ground pin of a known chip inside the machine, etc.
Check for continuity (_very_ low resistance in this case) to either
connection of the PSU connector. If one side of that connector is
directly connected to the logic ground line, then it's a _very_ good bet
it's the -ve side of the power supply.
-tony