Finding the Version by Central Administration
Open the Central Administration and go to the site settings page: Site Actions > Site Settings.

Finding the Version by PowerShell
Open PowerShell on the server and execute this query:
(get-item "hklm:software\microsoft\shared tools\web server extensions\12.0").getvalue("version")Finding the Version by IIS
Open the properties of a SharePoint site in IIS and switch to the HTTP Headers tab:
