My MacBook Pro has two graphics chips and automatically switches between them. How can I tell which one is currently in use? I'm just curious really; it's plenty fast enough whatever it's doing by default!
gfxCardStatus is a free app that lives in your menu bar. When you click it, it tells you which graphics card you're using. It also has a beta feature that lets you manually switch to one or the other.
Hope it serves you well.
You don't need to use separate software to check which graphics are being used.
Such information is provided by the System Profiler of OS X which you can access in two ways:
Only one GPU has the extra section "Displays, Color LCD". This GPU is the one that is currently in use.
e.g. In the first picture discrete graphics (ATI Radeon HD) are being used. In the second, integrated graphics (Intel HD 3000) are being used.