Every year I go through this. I see suspicious tiny strips missing from a corn leaf and approach the plant. Usually, the plant is still small enough to look down and peer inside the leaf-cupped top. The edges of the leaves are ragged and there are frass everywhere, so I pick up a grass stalk and push aside the tender young corn leaves inside the cup and ALWAYS there is a caterpillar, about 2 inches long. It's not difficult to goose the caterpillar in the tail end with the grass stalk and make it climb up from the narrowest bottom of the cup. Once there is clearance I just lift it out with the grass stalk much like the way snake catcher catches snakes. They are slow and stupid and will not make a fuss.
I've looked it up before and it's called armyworm. I don't usually find many that are affected, so far only two. Sometimes, I see the signs but no worm, and usually that means a Garden Patrol -- most likely a wasp -- has bundled it off to feed her babies.