When they start playing martial music on the TV/radio.
During the Cuban Missile Crisis, I blew it off as another failure of the Kennedy admin until I heard the first strains of a military march, followed by others. That's when I puckered up, went home, loaded the Springfield and waited.
After an hour I felt like an idiot and went back to the shop. (I was self-employed then.)
Funny thing. I did the same when I heard that Kennedy was assassinated. Thought it was some right wing whacko attempt to take over the govt and that LBJ would call up the militia.
WOW! I wasn't alive then, so I have to just imagine what those days were like. Thanks for the glimpse! I'd stand with you, though.