As soon as you play the game. The real test is how does a card performs in your system with the games you like to play. If you look at game play vs benchmark results things just don't add up. This again brings up the Quake timedemo which is a game specific demo. With this test you can not only test the speed of the card, but you can also look at the quality of the image.
Different cards produce different looks and feels to the game.
All of this is important to consider when judging a card. If they can they will. ATI, Intel, Nvidia, and VideoLogic have been accused of cheating. VideoLogic were optimizing their driver to work better in a particular environment. It was found that the same tweak would work in real world games too.