To identify if you have installed WSUS SP1;
You can check the version number for the wsusservice.exe file located in %ProgramFiles%\Update Services\service\bin\wsusservice.exe.
OR, check the WSUS Build number from WSUSAdmin home page (bottom of the page – Last line)
WSUS SP1 Build 18.104.22.16820
WSUS RTM Build 22.214.171.1242
WSUS RC Build 126.96.36.1990