gfxCardStatus is an unobtrusive menu bar app for OS X that allows MacBook Pro users to see which apps are affecting their battery life by using the more power-hungry graphics.
gfxCardStatus is free and open-source.
gfxCardStatus is localized in 20+ languages:
what the menu bar icons mean:
i — you're using the integrated GPU.
d — you're using the higher-powered discrete GPU.