The purpose of the question is to see how you'd handle a touchy situation with a superior. They also want to see if you can handle something like this discreetly or will you turn this into a spectacle.
Here's my answer: "Well, I would pull the Capt aside and say something like this: Look, Joe, if I am wrong about this, I apologize in advance, but I suspect you've been drinking and I think you should call in sick for this trip. At this point, if he has any sense, he will call in. If he doesn't, I will call the duty officer or the CP, but I'd give the guy a chance to do the right thing first".