rtejada,
To find the current AutoCAD version associated with "AutoCAD.Application", do the same thing Tony suggested, but instead of looking for (AutoCAD.Application.XX), look for (AutoCAD.Application). Then follow his instructions.
Basically what happens, is when you call CreateObject("AutoCAD.Applicaton") what ever the CLSID, default value, GUID that is defined there, will be instantiated.
If you want to see which versions are installed, look under
(HKLM\SOFTWARE\Autodesk\AutoCAD). You should notice that vertical products as well as AutoCAD will have subkeys under this, so you may need to filter the results based upon the "ProductName" in each subkey. Also note that just because a version is installed, it doesn't mean that the COM interface entries required to instantiate it are defined, you will still need to check using the method Tony describes above to verify that the computer knows which COM object to call.
C