There are different ways to view a source node while working in Action. For example, a 2-up view with Schematic and Source views allows you to view the complete scene and the interactive result of your source node.
From the View box (on the viewing/layout toolbar under the viewports), select Result, Front, and Matte views (grouped under Action Sources). Press F6 to cycle through these three views.
You can also the Sources Working view to provide you with the ability to view your source scene using a working camera, through which you can experiment without saving anything while preserving your camera setups. Each source node has its own working camera apart from the regular camera.
If you have multiple source nodes in your scene, you can use the Source View box (located at the bottom of the Action menu buttons) to select which source to view in the image window.