window.google.maps.Map is not the map object, it is the constructor of said object. The object itself is in another castle (variable).
obj instanceof google.maps.Map
should return true if the object is a googlemaps Map object. If you really can't find the variable you can write a loop that goes through all objects on your page recursively to find the object that returns true to this line.
As a side note in the chrome debugger the Map object appears like this:
It says that the map type is "em" which is rather odd, this is because the GoogleMaps library is minified