How can I check if my video card is overheating?

I play Quake Live and it's fine but then after a while all the graphics become messed up, I get these black triangles emanating from my cursor that cover the screen and I can't see anything. I was thinking maybe my graphics card is overheating. Is there anyway of monitoring this, or some software that puts it through it's paces and informs me of any malfunctions?|||You can try speedfan. But if it is an aftermarket video card you might need a bigger power supply.