When is a sign from God?

I was re-reading Marilynne Robinson's luminous 2004 novel "Gilead" on the plane the other day, and ran across a passage that made me wonder about something. The novel is written in the form of a book-length letter from an aged Iowa Protestant pastor to his young son. It's a history of the family and a guide to life from a father who knows he will be dead by the time he's old enough to have this conversation with his boy. Anyway, here's the passage in question. The pastor is writing about the Spanish flu epidemic of 1918-19:

It was a strange sickness -- I saw it over at Fort Riley. Those boys were drowning in their own blood. They couldn't even speak for the blood in their throats, in their mouths. So many of them died so fast there was no place to put them, and they stacked the bodies in the yard. I went over there to help out, and I saw it myself. They drafted all the boys at the college, and influenza swept through there so bad the place had to be closed down and the buildings filled with cots like hospital wards, and there was terrible death, right here in Iowa. Now, if these things were not signs, I don't know what a sign would look like. So I wrote a sermon about it. I said, or I meant to say, that these deaths were rescuing foolish young men from the consequences of their own ignorance and courage, that the Lord was gathering them in before they could go off and commit murder against their brothers. And I said that their deaths were a sign and a warning to the rest of us that the desire for war would bring the consequences of war, because there is no ocean big enough to protect us from the Lord's judgement when we decide to hammer our ploughshares into swords and our pruning hooks into spears, in contempt of the will and the grace of God.

The old pastor is a pacifist, hence his reading of the "signs." What stayed in my mind was this line: Now, if these things were not signs, I don't know what a sign would look like." I thought: How would we determine a sign today? Can we only determine them in retrospect? When is some event or series of events properly read as a sign from God? And how can we determine what it means in terms of causality ("If you don't do this, then that will happen.")

Thoughts?