A. A. When a Service Pack is installed using the normal method (e.g. not just copying the files to a build location) the service pack version is entered into the registry value CSDVersion which is under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion.
The value is of the formal "Service Pack n", e.g. "Service Pack 4" but can have extra information if it is a beta or release candidate, e.g. "Service Pack 4, RC 1.99".
To check this from the command line you could use the REG.EXE Resource Kit supplement 2 utility:
C:\>reg query "HKLM\SOFTWARE\Microsoft\Windows
REG_SZ CSDVersion Service Pack 4, RC 1.99
Make sure you put the value in double quotes (").
An alternative is to just run WINVER.EXE which will tell you your current build and Service Pack version. You can also use WINMSD.EXE or Help/About in Explorer.