i would loop the unigine valley benchmark for about an hour on its extreme hd preset(though change the resoluition if your not on a 1080p monitor). after an hour watch it for a bit and see if you can see any artifacts.
use msi afterburner/gpuz/etc to log the stats and most importantly gpu temps. check temps with a few online reviews to make sure they are not drastically different which could be a sign of a bad factory thermal paste application or another problem.