Determine if in theme preview

Is there a way to determine if the user is seeing the index.php page in the Theme Preview window or for real? My theme build completely dynamic pages based on options set on those pages. During the preview, none of those options have been set so I see a blank page.

I would like to know if the theme is being previewed so I can create a "dummy" page on the fly to show the user a sample of the type of page he can create.

Thanks in advance.