You can't.
There's nothing about a PSU that makes it able to power an overclocked system or not... The PSU just has to be able to power the system, and the lower its ripple and the tighter its voltage regulation, generally the higher you'll be able to overclock (up to a point). Basically you just need a quality PSU.
As for telling whether a PSU is quality or not, that's a whole different story.
There's nothing about a PSU that makes it able to power an overclocked system or not... The PSU just has to be able to power the system, and the lower its ripple and the tighter its voltage regulation, generally the higher you'll be able to overclock (up to a point). Basically you just need a quality PSU.
As for telling whether a PSU is quality or not, that's a whole different story.