How to determine which Office SP installed

Office 2003:

Start any Office application, such as Word. Click Help -> About Microsoft Word

RTM release - 11.0.5604.0 and higher
SP1 - 11.0.6255.0 and higher
SP2 - 11.0.7969.0
SP3 -  11.0.8173.0

Office 2007:

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

RTM release - 12.0.4518.1014
SP1 - 12.0.6211.1000 and higher
SP2 - 12.0.6415.1000 and higher

Offcie 2010:

Start any Office application, such as Word. Click on File -> Help -> Version:

RTM release - 14.0.4763.1000
SP1 - 14.0.6029.1000
SP2 -   14.0.7015.1000