What's the foolproof way to tell which version(s) of .NET are installed on a production Windows Server?
You should open up IE on the server for which you are looking for this info, and go to this site: http://www.hanselman.com/smallestdotnet/
That's all it takes.
The site has a script that looks your browser's "UserAgent" and figures out what version (if any) of the .NET Framework you have (or don't have) installed, and displays it automatically (then calculates the total size if you chose to download the .NET Framework).
You can programmatically check the registry and a few other things as per this blog entry.
The registry key to look at is
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\...]
Found answer from here:
Check which .NET Framework version is installed
Open Command Prompt and copy paste one of the below command lines
dir %WINDIR%\Microsoft.Net\Framework\v*or
dir %WINDIR%\Microsoft.Net\Framework\v* /O:-N /B