I've found an extended Heaven Extreme Tes run (like 30min or so ) with max AA along with Vantage (max res, settings, aa, resolution pending monitor ) with feature tests 3x loop and 3DM11 on demo + test usually shows instability. There are never any guarantees with this stuff but in my own experiance if a voltage / clock is unable complete all these tests, instability is likely. From there I simple do the things I normally do ( games, decoding / encoding etc. ) and if all is well you have your answer

On a side note I've found Just Cause 2 to be quite effective at stressing hardware and ironing out instablility. Sadly There appears to be some inherent driver instablilty and it has ocassionaly crapped itself on me on more than 1 machine ( another with a 470 ) even at stock.