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Intel burn test.
Or if you're a sadist, linpack Xtreme.
As a quick test bf1 or BFV quickly show instability in overclocks, if it is stable for an hour or more you are heading in the right direction. I have had my machine pass hours of realbench and memory tests but fail fairly quickly on bf1 / bf5.
As above it depends what your intended usage is.
The best test is just using it normally. If you get a crash, bsod or whatever then tweak more. Don't spend half your life benching and running CPU tests.
Trying to find your highest clock without using quick stress tests, from which to know what to back off from, would be a right pain.
Normal use is then the final decider, but if I find through that I'm not stable, then one notch down will usually do it.
I use OCCT.