plug in the card and the pci ex power connectors, a monitor on any of the outputs and if you get to the boot screen it's working. (starting at least). after that in windows you should install the latest catalyst drivers from amd.com and that will test the 3d part of it
next comes additional stress testing with furmark for example for a few hours and monitor temps - in catalyst of hwmonitor - during the test to see that everything is in order.