You can use the VRNotifications component to check if the user has removed their headset, as explained by st.shaeen.
But you can also use the “Get VR Focus State” to get the same result. On Windows 7 the “Get VR Focus State” node will also return false when the Oculus menu is up, but if you’re running on Windows 10 you’ll need to use the “Has System Overlay Present” node in order to trigger a pause when the Oculus menu is up.
Please note that these features will only work for the Oculus and not the Vive.