void Update() {

        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if (Physics.Raycast (ray, out hit, 100)) {
            Debug.Log ("Hit something!");
        } else {
            Debug.Log ("No Hit something!");
        }
}

Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);