is there a cardboard/VR emulator available?
Just run the app. The Google Cardboard is just a way of attaching the lenses and phone to your face. The phone doesn't know whether it's in the headset or not. The app just displays the two views on the screen, whether it's in the headset or not. 



Note that this only applies to Google Cardboard apps, not to Samsung Gear VR. Gear VR has its own electronics supplying extra sensor data to the phone, so apps written to work with Gear VR will only work with an actual Gear VR headset.