http://msdn.microsoft.com/en-us/library/ff721969.aspx

Go to the registry key:

Regedit

HKEY_LOCAL_MACHINE -SOFTWARE-MICROSOFT

   SHARED TOOLS-Web Server Extensions – 14.0

           WSS-Installed Products

Then, match the key that is installed in the list below.

              

“BEED1F75-C398-4447-AEF1-E66E1F0DF91E”, “SharePoint Foundation 2010”
“1328E89E-7EC8-4F7E-809E-7E945796E511”, “Search Server Express 2010”

“B2C0B444-3914-4ACB-A0B8-7CF50A8F7AA0”, “SharePoint Server 2010 Standard Trial”
“3FDFBCC8-B3E4-4482-91FA-122C6432805C”, “SharePoint Server 2010 Standard”

“88BED06D-8C6B-4E62-AB01-546D6005FE97”, “SharePoint Server 2010 Enterprise Trial”
“D5595F62-449B-4061-B0B2-0CBAD410BB51”, “SharePoint Server 2010 Enterprise”

“BC4C1C97-9013-4033-A0DD-9DC9E6D6C887”, “Search Server 2010 Trial”
“08460AA2-A176-442C-BDCA-26928704D80B”, “Search Server 2010”
“84902853-59F6-4B20-BC7C-DE4F419FEFAD”, “Project Server 2010 Trial”
“ED21638F-97FF-4A65-AD9B-6889B93065E2”, “Project Server 2010”
“926E4E17-087B-47D1-8BD7-91A394BC6196”, “Office Web Companions 2010”