Install MSI afterburner and isolate component (core or memory), bumping clocks up (maybe 20MHz) until you see artifacts to find max good clock. Test for artifacts with MSI Kombustor/Furmark. Repeat with other component. Then reduce max clocks by 5% and see if it's stable with both components on high OC. Months of playing games will tell you if it's 100% stable.
Immediately exit tests if you see memory artifacts since they can do permanent damage more so than core/shader artifacts.
Monitor your temps and don't overvoltage. You might break it, not that I've done that yet. Either way, it's not my fault if you do.