Does IVssItem.IsDifferent work for you?
http://msdn2.microsoft.com/en-us/library/microsoft.visualstudio.sourcesafe.inter
op.ivssitem.isdifferent(VS.80).aspx
You have to get a VSSItem for the item in the VSS database, and call
IsDifferent to compare it against the local version.

If you want to display a visual diff of the files, "ssexp /diff" might be
what you're looking for
http://msdn2.microsoft.com/en-us/library/ms181130(VS.80).aspx
You have to use first IVssItem.Get to get a local copy C:\temp\file.tmp of
the item from the VSS project $/VssProject/File.txt, then run ssexp in diff
mode.
E.g: ssexp /diff C:\temp\file.tmp C:\MyProject\File.txt
$/VssProject/File.txt