U can check the card in several ways...
1st check it physically that there isnt any physical damage like IC repair.....or deep scratch on the PCB..
secondly check the fan if it spinning properly..
and third once u have the card and it giving display..
either use furmark or msi kombuster and do a stress test for 10 mins..
and check the temps and no visual artifacts and least but not the least..
use GPUZ and check the actual cores and and mem speed to determin that the card is not altered in anyway....simple