I'm not familiar with Apple power supplies, but probably there is different wirings than in AT or ATX power supplies. Sure my last tip about draining capacitors will work just fine with it, but you need to look information about the wirings. My previous posts above about the wirings applies only into AT and ATX power supplies.
If it's very old, I mean something from from the first half of 90's, then same trick might apply as for AT power supplies. If it has hardware power switch, then try it all wires disconnected. If it starts running, then just measure it's +12V voltage and you should be fine, but if it doesn't start running, then try to add some load into +5V and start it again. If it starts now, then measure +12V and you should be fine. If these don't work, then I cannot help you. In this case Google might be your best friend for finding information about Apple power supplies.