Use raycast to see if something else than terrain is under

Hi! I have a script that instantiates an object that I need to not instantiate if there already is something else than terrain where I instantiates. The most logical solution I think would be something using a raycast, although I'm not certain how I would check if there is something else than terrain where I instantiates...

Here is the instantiation code

 if(Input.GetKeyDown(KeyCode.Mouse0)){
             Debug.Log ("Instantiating Windmill.");
             GameObject newObject;
             newObject = (GameObject) Instantiate(Object1, hit.point, Quaternion.Euler (0,120,0));     
             Vector3 currentPosition = newObject.transform.position;     
             currentPosition.y = currentPosition.y + 500.0f;     
             newObject.transform.position = currentPosition;
         }

Thanks in advance!