how to get MouseLook to stay strictly in either X or Y planes

Hi, I am trying to modify the MouseLook script to force the mouse motion to stay strictly in the X or Y planes rather than be able to tilt. Here's my code:

 if (Input.GetAxis("Mouse X")){
         transform.Rotate(0, Input.GetAxis("Mouse X") * sensitivityX, 0);
     }
     if (Input.GetAxis ("Mouse Y")){
         transform.Rotate(-Input.GetAxis("Mouse Y") * sensitivityY, 0, 0);
     }

Of course this doesn't work because Input.GetAxis returns a float and I need a boolean value. Is there any way to address Input.GetAxis to find out whether it is using Mouse X or Mouse Y at any given moment. Thanks.

Zaffer