How do I determine which Office 2007 service pack is installed?

1. Start any Office application, such as Word.
2. Click on the Office Menu (upper left)
3. Click "Word options" at the bottom of the menu
4. There, select "Resources" on the left

5. The version number will be displayed alongside the "About" button on the right:

   

Official answer from MS: is not that easy.

In any Office product, click on Help | About.

Um, just realised there's no Help menu in Office 2007, which may well be why you were asking. One product that does still have the normal menus is Outlook 2007, so you can do it there. The about screen on mine says:

Microsoft Office Outlook 2007 (12.0.6504.5000) SP2 MSO (12.0.6425.1000)

which is Service Pack 2.

In Word, Office button, Word Options, Resources (and then you can see the version number at the bottom)

I got the current product number (from Johannes Rössel's technique), then compared to this (from ianix's link):

Original: 12.0.4518.1014  
SP1:      12.0.6213.1000  
SP2:      12.0.6423.1000   
SP2:      12.0.6562.5003 (from 1/08/2012)