Start Central Administration, Click on Operations, then Servers on server.
Without SP1, it's version:
12.0.0.6020
With, SP1, it is version:
12.0.0.6219
***Update 6.12.08 ***
Check out
http://support.microsoft.com/kb/948945
And look at the file sizes, dates, and version number to see if you've updated to WSS 3.0 or not.
Also, this KB article: http://support.microsoft.com/kb/936984
Contains a specific section on determining whether or not you've upgraded to MOSS SP1 on the third link down under Installation details. So there you go.
***End Update***
