Is there a way to find out which objects are rendered at a certain time?
My problem is that the statistics show some 16000 vertices even if I point the camera up in the sky where there should be nothing and the visible bounding from Debugger are out of the frustum as well.
What about debugging? 
Point your camera in the sky, add a breakpoint in the render method and step through the draw methods - you can easily see what is rendered and why.