How do I tell if .NET 3.5 SP1 is installed?

How can I find out if SP1 has been installed on a server which has .NET 3.5?

Look at HKLM\SOFTWARE\Microsoft\NET Framework Setup\NDP\v3.5\. One of these must be true:

• The Version value in that key should be 3.5.30729.01
• Or the SP value in the same key should be 1

In C# (taken from the first comment), you could do something along these lines:

const string name = @"SOFTWARE\Microsoft\NET Framework Setup\NDP\v3.5";
RegistryKey subKey = Registry.LocalMachine.OpenSubKey(name);
var version = subKey.GetValue("Version").ToString();
var servicePack = subKey.GetValue("SP").ToString();

You could go to SmallestDotNet using IE from the server. That will tell you the version and also provide a download link if you're out of date.