download furmark, download an overclocking software ( i think you have one but anyways)
step #1 increase the core clock in 10-20 increments. - step #2 run furmark ( any setting really ) run it for a moment, <1 min is enough, try to see graphical failures, screen tearing, errors in the motion etcetc, if no graphical errors are shown repeat step #1. when you get an graphical error lower your core clock by 5 and do step #2 until you have no graphical errors.
congratz, you have reached your default voltage max core clock.
and keep eye on the temps, dont let them get above 75c