wpTrackerCS

If you have Windows 8, and use the built-in wallpaper slideshow, the wpTracker might be useful to you. If you have multiple monitors and you want to know which wallpaper changed last, and which will change next, then wpTracker was made for you.

wpTracker is a really simple program that consists of a notification icon which shows a number to tell you which monitor had the wallpaper changed last. If you put your mouse over it, it’ll also give you a tooltip telling you when the wallpaper last changed.

Once you've run wpTracker, you should see an icon in the system tray that looks like a screen with a number on it. The number indicates which monitor's wallpaper changed last. This icon will automatically update, by default, every 30 seconds, however this can be changed in Options.