This Daz Studio script adds a light to your current view that simulates the preview headlight and sets all your other lights to invisible. This will allow you to quickly inspect your shaders, bump and displacement maps with a fast 3Delight spot render. (the Daz preview headlight will not render when you have lights in your scene, this one will).
Run the script again to remove the headlight (if it’s added to your perspective view it won’t show up in the scene tab).
You will need to turn your other lights back on manually with the eye icon in the scene tab.
The intensity is a bit higher than the Daz headlight. If you want to change that the code is already in the script, just uncomment the marked line.
With thanks to cwichura for the idea
<Edit> Update 18-03-2013 : Running the script again to remove the headlight simulator now also turns your other lights back on.
I have no idea why my brain didn’t include that from the start