EVGA GeForce GTX 760 Superclocked w/ EVGA ACX Cooler

Play a game, or do a benchmark like unigine valley or 3dmark. If scores or fps match up to common internet scores, then it is working correctly.

good programs to monitor performance are gpu-z, fraps, and msi afterburner. Just make sure your card is running within it's quoted specs and you can then be sure you are doing fine.