Detect whether marker is present or not

I have a marker and a ball is displayed on it using augmented reality and vuforia on unity. This is my first project and I need help please guys.What I have to do is when the marker is not visible to the camera,as in I move it out of the camera's view the ball should also disappear but the ball stays there.I am new to c# but I know c and c++.

Thanks a lot :)