1. Open Registry Editor (by typing Regedit in Start Search or by going to a command prompt and entering regedit)
2. Navigate to the following registry key:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion
3. In the right pane, you will see lots of registry subkeys. Check the following Key
BuildLabEx
This contains build and revision number. For example, Vista SP1 Refresh 2 (the purported RTM) has the following build number: 6001.18000.rtm.080118-1840
This is what I have - installed as SP1 RC Refresh 2 of course. But some circles are suggesting that RTM is build 2040