Reliable way to test suspicious PSU?

[Dangerous_Dave]

I was rooting around my stuff and found an oldish 1200W Cooler Master Silent Pro PSU that I bought off Ebay a couple of years ago. IIRC I tried to return it, but ended getting back my money without having to send the unit, so here it is now. I can't remember if I started the return because it looked like it had been opened/fiddled around with or because it actually wasn't working. Is there any reliable way to test the functionality of the PSU without connecting it to an actual system and potentially damaging things? I've seen some 'PSU testers' on Ebay.

Thanks in advance.

 

[Dangerous_Dave]

Well, I picked up one of those PSU testers. It's beeping happily away at me now. All the voltages look ok with the exception of the -12V which is reading -11.4V. Everything else is within 0.2V of the right number.

How comfortable would you be to go ahead and use this PSU? I'm sure somebody's opened it up, but I have no idea why. On the other hand, it's a fairly valuable item to just throw away.