In this directory is a file named "ZopeExtEditDummyOCX.ocx".  The OCX
does nothing whatsoever.  But if you install it on a client system, it
can be used to test for the presence of external editor.

To install by hand:

regsvr32 ZopeExtEditDummyOCX.ocx

To install via Innosetup see "Installing OCXs" at
http://www.jrsoftware.org/iskb.php?a=vb ... I haven't tried it yet tho

Once it's installed, you can use the folllowing VBScript to do the
detection:

<SCRIPT LANGUAGE=VBScript>
 return IsObject(
 CreateObject("ZOPEEXTEDITDUMMYOCX.ZopeExtEditDummyOCXCtrl.1"))
</SCRIPT>

Or an x-platform bit of Javascript that works under Moz or IE to do same
is:

function externalEditorInstalled() {
     // mozilla: life is easy; test for mimetype availability
     mt  = 'application/x-zope-edit'
     if ( navigator.mimeTypes && navigator.mimeTypes.length ) {
          thing = navigator.mimeTypes[mt];
          return thing;
     }

     // IE: test for presence of dummy OCX installed by installer
     else if ( ActiveXObject ) {
       return new
ActiveXObject("ZOPEEXTEDITDUMMYOCX.ZopeExtEditDummyOCXCtrl.1");
     }

     // others: return true for conservatism's sake
     else {
          return true;
     }

}          

... then condition the visibility of "pencil icons" (or whatever) on the
result of that function ... I have been wrapping the result of all
occurrences of ExternalEditLink in a span that has a CSS class named
"eelink" which has style "display:none", then I have a bit of javascript
at the bottom of templates that show these icons which calls the
"externalEditorInstalled()" function; if it returns true, it iterates
over all of the "eelink" elements on the page switching them on.