You can check registry (HKLM->Microsoft>SharePoint Portal
Server->Current Version) should be 10.145.4629
Or just check some files, which are specific to SP1, like I do :)
For example, check in SPS 'bin' directory file 'portalperf.vbs", which
is a sample implementation of new dashboards/workspaces caching
mechanism
Obviously, you probably 'd implement it as a small bit of code in
WebPart or just script, which do this check.