Do you have a monitor hooked to the external card?
Found on google:
Graphics acceleration can be verified through the following (non-exhaustive) actions:
- Translucent Finder's bar (most visible from Snow Leopard to Mavericks)
- LaunchPad through Dock icon (clicking in and out of LaunchPad should be quick and totally smooth)
- DVD player app -> Help Menu -> Show Supported Features (all should be enabled)
- Desktop switching through Ctrl-Up/Ctrl-Down/Ctrl-Left/Ctrl-Right (should be quick and smooth)
- Desktop and Screen Saver PrefPane (all desktop pictures and screen savers should show Ok)
Without graphics acceleration, system will feel laggy and graphics glitches will occur.