Try using this on an object pre-placed in the mission editor such as a house:
hint typeOf _object; // Shows the class name (XXX)
Now place an object in the editor, save the map, and open up the Mission.sqm. Find the line: vehicle = "XXX", and replace XXX with the name of the object that you found above. Save it, and load the map. This method is no longer available since Armed Assault.