we don't know either, carol. only god does...
i am sorry that you and your family had to make the hardest decision at ICU for dad. it seems like you have done the best you can, but your best is simply not good enough to bring a life back.
technically, there are signs and symptoms that we assess when death is near. following the patient's wishes by their advance directive gives directions how the medical treatment will go. "what would mom (or dad) do if she/ he was capable of making a decision for herself/himself?"
the next question is how the prognosis is like? most doctors i have worked with have no problems relaying the depth of disease and the condition to the family, but i do think its the family that sometimes do not understand how aggressive the interventions can be. this explains why, in this episode of 60 Minutes, doctors simply do not put a defibrilator in a terminally ill patient.