George's post above is so helpful, and his related post goes along with it: Who is to say when someone is in trouble?
I especially like this part:
I found a guy on the switchbacks. When I first talked to him, he seemed fine -- for about 2 minutes when he started talking about his wife's garage sale on the summit. That's called a clue.
(For those who may not know, Mr. Durkee is a backcountry ranger with Seki -- Sequoia/Kings Canyon National Park.)