How to find public key token for a .NET Framework DLL or assembly

For example if you are looking for public key token of System.Web.dll of .NET Framework 4 then go to the Config folder of the Framework (normally it is C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Config), open machine.config in a text editor and look for soemthing similar to

sectionGroup name="system.web"

If you find this line then look for "PublicKeyToken" property at the end of the same line.