first thing that comes to mind is ati tool http://www.majorgeeks.com/download.php?det=4109
im not sure weather it matters if your card is nvidia or ati, (works with my nvidia card). also depends how old the card is aswell.
once installed there is a "scan for artifacts" button that puts your card through a kind of stress test. thats a sure way to see if its working or not.
its a prime95 for your graphics card, and tests it just as much as a modern game would do, if its in good shape it should pass without errors, but errors are not fatal as such. it halts on all errors i think