Using Central Administration Web Site
Operations > Topology and Services > Servers in farm
Using SharePoint API's (PowerShell)
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")
$Farm = [Microsoft.SharePoint.Administration.SPFarm]::Local
$Farm.BuildVersion
Using HTTP Server Response Header
The header "MicrosoftSharePointTeamServices" includes the build Version
Using Registry
Path: HKLM\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\12.0\Version
Using SQL Query Against SharePoint Configuration Database
SELECT [Version] FROM [CONFIGDBNAME].[dbo].[Versions] WHERE VersionId = '00000000-0000-0000-0000-000000000000' ORDER BY Id DESC
Important
Do not rely on Control Panel or WMI class WIN32_Product. They relfect the build number the server was originally deployed with and does not take into account subsequent updates nor updates deployed from the "Updates" folder (slipstreamed installation).