How is it possible to check if MOSS Standard or MOSS Enterprise is installed?
This link should sort you out...
Determining sharepoint versions
Edit: The article has now disappeared, so see Dan's answer below
From code, an easy way to check MOSS license is to see if the "Premium" (Enterprise) Farm-scoped features are installed in SPFarm.Local.FeatureDefinitions
:
99ee0928-7342-4739-865d-35b61ea4eaf0 BDCAdminUILinks
e4e6a041-bc5b-45cb-beab-885a27079f74 ExcelServer
a573867a-37ca-49dc-86b0-7d033a7ed2c8 PremiumSiteStapling
a10b6aa4-135d-4598-88d1-8d4ff5691d13 ipfsAdminLinks
cdfa39c6-6413-4508-bccf-bf30368472b3 DataConnectionLibraryStapling
Since the link given in Paul's answer seems to require sign up at ASP.NET weblogs, here's the answer without having to go away and log in elsewhere:
Central Admin -> Operations -> Upgrade and Migration -> Convert license type
The 'Current License' field will tell you. Once an enterprise license key has been entered, you also need to enable the enterprise features by visiting:
Central Admin -> Operations -> Upgrade and Migration -> Enable Enterprise Features
Once this is done, you can't revert the installation back to standard features.