To test the onboard graphics you would first need to make sure it is enabled in the BIOS, then shut down the PC and connect your monitor to that port. Then boot up and it should be detected. If you don't see the POST status on the monitor when you boot, you may need to pull the graphics card. Otherwise, you could disable the graphics card in device manger after it boots ok.
For a really clean test, you might want to uninstall the driver and software for the graphics card, and then pull the card from the PC, so that you would only have the onboard graphics enabled, and no extra drivers lingering about.
Windows should automatically detect and install any drivers for the onboard graphics, but to be sure you could go out to your PC or motherboard site and download the latest drivers for the onboard graphics from there.