Seek and ye shall find ;-) It's a good idea to have a couple of tomato plants growing away from the ones you wish to harvest. When you find the hornworm(s), place them on those plants.

Here is a link that might be useful: This insect is parasitized by a number of insects

I second Terramadre..
I raise Butterflies and Moth and I am raising a Tomatoe Hornworm right now.. I wish I would find more..I have tomatoes planted for us to eat and then some more next to my Butterfly-Garden..
If you find the BUGGER,contact me, I would love to have him..
Butterfly-Greetings
Susanne(SUE)

I've noticed that sometimes I blame tomato hornworms when its really blister beetles. They eat tomatoes faster than hornworms if that is possible. We take a stick and knock them off into a jar of rubbing alcohol.

why not use some good ole fashioned sevin dust and kill it

Do you really want to put Sevin dust on a plant that grows things that you EAT?