That's a difficult situation and I would say largely dependent on the Crew you are jumpseating with. I always brief my jumpseaters that if they see anything that looks unusual or unsafe then to please bring it to my attention. I have flown as an enlisted crew member in the Air Force and as a flight engineer and I understand how having a third pair of eyes overlooking the big picture can greatly increase safety. I would say if you feel the safety of the aircraft is in jeapordy then you are completely justified in calling a go around.