i can't see any apps utilising app thinning at the moment.
Is there a list of apps that use this?
No list that I am aware of and this is not something that will be called out on the application page. This is something that will happen as a part of the install. When the device requests the download from the store the device ID will tell the store to send a copy specific to the device. What you get is an application with resolutions and related data that can be used by the device versus an app with every device configuration included. The iPhone version of an application does not need the images from the iPad version for example. You might know this but others reading might not so I stated that here.
If you want apps that are thinned or potentially thinned, you will need to reinstall your apps. This is not something that will happen just because. Maybe over time as updates occur your apps will be reduced in size during the update but as far as I know your current option is only to remove and reinstall apps and in the process they will be thinned.