Can iTunes reveal which songs are currently on a device? How can I get iTunes to show at a glance which items are already on my iPhone.

I'd like to add some new tunes to my iPhone, but find myself adding songs that are already on the device.  I start looking through my collection, but I can't see what's already on my iPhone. I'd like to be able to see what's not currently synced to my phone, so I can focus on adding them. 

On my Zune, all synced items were marked with a little icon so you could see if it was already on the device. Like so:



The only place where summary information of what has synced is this following screen. (But obviously, it's a really ugly and unpleasant way to browse your music!)
As I understand your desired functionality, iTunes does not adjust the main library view based on a connected device. You have to select the specific device and see what it contains. 

The closest you can get is to plug-in your iPhone into your computer so it appears under Devices in iTunes. If you click on the little arrow in front of your iPhone's name, you can browse its content easily.



In addition you can also see which playlists, artists, genres and albums get synced in the  Music section of the syncing window.