You can use gpu-z for that. GPU Load is on the sensors tab, assuming your GPU and drivers support it (all modern ones do).
Then, if you notice rather unusual GPU load when you just idle on desktop, you can use gpu shark to see which all processes use the GPU. Note: If it is a virus that's causing the gpu load, it may not be listed, if the virus manages to hide itself well enough. Also good to remember, most browsers do make some gpu load when you browse, since almost all browsers use gpu acceleration these days.
Edited by Microx256 - 10/27/13 at 11:01pm