I have just purchased a 2nd hand graphics card. What is the best way to stress it to test it works properly?
1) Inspect for visual damage. 2) Insert in system with appropriate slot 3) Run Furmark. If the card survives step 3 it'll survive anything
Assuming you bought it to game on then simply play some games on it. Furmark is a good way to test a GPU to its absolute limit although it isn't that representative of in game loads.
Furmark will find out for sure whether there is anything wrong with the card. It's also known for killing cards with inadequate cooling, though, such as stock AMD HD 48xx cards