Our first clue that an ant swarm was in the vicinity was the excited calls of the Bicolored Antbirds. We have never seen an ant swarm in Panamá that was not accompanied by several individuals of this species. Ridgely, in "A Guide to the Birds of Panamá" says that the Bicolored Antbird (Gymnopithys leucaspis) is "An inveterate follower of army ants and rarely seen away from them; in such assemblages, it is usually the most numerous of the several attendant species."
My second clue about the ant swarm was being bitten on the tops of my feet by a couple of ants. I was standing at the edge of the road in my flip flops, and the ants were coming out of the forest to cross the road right across my feet. After a few seconds of foot-stomping, I returned to the truck and put on some shoes and socks. Fortunately, the bites of these army ants produced a mild stinging sensation for only about five minutes, and that was the last of the effects (except that I now look down at my feet a little more often when I'm standing still for any length of time.)
Marco shot some video of the more cooperative individuals with this swarm: