For WSS, go to the SharePoint Central Administration page, on the
left of the page click Windows SharePoint Services > Configure
Virtual Server Settings...on that page it will list each virtual
server and the version number. You will see either 6.0.2.5530 or
6.0.2.6361, the latter being the version number for SP1. I'm not
exactly sure of how to determine this for SPS,
it's been a few months since I installed the service packs. Just
keep in mind that once you install, you won't be able to restore any
pre-SP1 backups.