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Horn worms are very difficult to find during the day, as they camouflage very well against the leaves and stems of the plant they are eating. They glow somewhat under UV/Blacklight so with an flashlight, you can locate them at night.
What is needed
I use the TaoTronics UV Flashlight Blacklight. It emits a 395 nm wavelength throught 12 LEDs. There are many other brands of different sizes available, but this one seems to work well and is very inexpensive.
Wait until it is completely dark. Then walk around your plants, shining the UV blacklight on the leaves. The leaves will appear dark, and any hornworms present will glow.