Play some games on it, try a few different ones, try one that isnt too graphically intense, and a few that when you max them out they will stress the card hard, that will help you find out if its artifacting pretty quickly.
http://www.majorgeeks.com/downloadget.php?id=4183&file=... Use that, set to stress test, run it about an hour, if you want to be VERY VERY safe, do 2 hours. That much stress on a card for a long period of time is not good. After that, run a few graphics intensive games. If you have no crashes or bad signals, you are fine. Hope I helped.