My method of choice has always been to look at the Windows folders listed under C:\Windows\Microsoft.NET\Framework.
Each framework version has its own sub-folder:
14/03/2011 23:11 <DIR> v1.0.3705
14/07/2009 03:37 <DIR> v1.1.4322
14/03/2011 23:11 <DIR> v2.0.50727
14/07/2009 05:56 <DIR> v3.0
14/03/2011 23:11 <DIR> v3.5
28/04/2011 14:44 <DIR> v4.0.30319
You also can easily get this from a command prompt by dir %windir%\Microsoft.NET\Framework\ /AD.