I personally use a combination of 3D Mark then Furmark. 3D Mark costs money for anything beyond the most basic benchmark, which I think is fine and Furmark is free. Furmark however, is a bit controversial in that it generates a graphics load that's unrealistic in normal gaming. Whereas gaming you'll see the graphics card go from periods of high load to low load, Furmark keeps it at a high load, which is a lot more stressful. That said, if your graphics card has no issues for a few hours of that, you're pretty much good to go.