You will know it if a tomato hornworm is at work in your tomato patch, because your beautiful plants will suddenly begin disappearing before your eyes!  These caterpillars are huge - 3-4 inches when full grown, and are a light green with eight diagonal usually silver-toned markings - overall making them blend well with tomato foliage. These caterpillars have a green or reddish horn protruding from their back ends (thus the name hornworm).  They are voracious eaters and in late summer will even feed on the tomatoes themselves, doing serious damage to both the plant foliage and the fruits. 

The best defense for the home gardener is to keep a watchful eye on your plants, and hand pick any hornworms you find.  Preventative measures can include companion planting, with Dill, Borage,  Marigold, Petunia, Pot Marigold,  and Basil (especially Opal Basil) all widely accepted companion plants for thwarting of tomato hornworms.  Environmentally safe insecticides such as Bt (most commonly sold as Dipel) can be used in severe cases, but this is usually not necessary in a home garden situation, and any insecticides should be used only as a last resort.