A bore scope camera is the way to go to see inside. If any of the plugs are damaged you'll know the piston probably bent a valve and the valve got pushed up against the spark plug. If the plugs look ok, then your head may be ok. I think if you pull the valve cover, this should expose the top of the timing belt to see if it broke. I can't remember the layout of these engines off the top of my head, There may be a plastic cover over the timing belt too.