Actaully, there is a way to do this but it is not 'user firendly' and may not
be documented either. There is an installer registry key that shows where the
software was installed from, but it may not be what people expect.

The information would mostly refer to the Local installer cache location
rather than the source media. This is because the Office installer
essentially copies the install files onto the system drive before actually
installing. The registry remembers that cache location rather than the
original source media path. In the case that you installed without a cache it
would show the source media path (which still does not tell you anything
about whether a GPO was used or not).

The big problem with the information is that it is indexed by product ID
guids rather than names so it is pretty user hostile. If you are curious pop
open regedit and look at this key and its subkeys:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products\00002109110000000000000000F01FEC
(this is Microsoft Office Professional Plus 2007, but the IDs under
Products represent Product IDs so you can look through them and find the app
you need).

Naturally you need to be very careful in here or you may hose your system
badly...