You can take screenshots and even record video of any app on your device. For doing that you need to buy Video Capture Card (HDMI -> USB) and than it will connect to your PC as webcam. This is good think for demo and mobility (You can test your app even without an external monitor or TV) just with your laptop.

But you cannot take screenshots of apps except dev one. If this is your case and you want to take a screenshot of your dev app you need to open device ip in browser and in Tools select create a screenshot.