Tomato hornworms are voracious feeders and older larvae can do quite a bit of damage to tomatoes and related crops. These older larvae are easy to identify by the stripes on their sides and their characteristic horn at their tail-end, but you may also see signs of feeding and conspicous piles of "frass" (aka insect poop).
Best practice for control is physical removal. Finding young larvae is key to avoiding crop damage. The caterpillars are smaller (and so are their poops) but they still have that characteristic horn. Products containing Bacillus thuringiensis (Bt) labeled for caterpillars are effective on this pest, but only when they are young.