BT for a hornworm? That really seems like overkill to me.
I just take a paper towel, pick them off the plants, and either throw them in the woods, or if they've pissed me off by eating a pepper leaf to the bone, down the toilet. If I don't have a paper towel on hand, I just pull them off with my hands and chuck them. They are totally harmless. A little squishy but not in a beyond-gross way.
There are two ways to know you got 'em. Your leaves get ravenously consumed, very quickly - much more quickly than with other pests, and also they leave telltale droppings that look I guess like little round, brown pellets. You will typically find a group of the pellets - about 5 to 20 of them. The worms are to be found very near directly overhead of the droppings. If you see 5 droppings, you're in good shape. If you see 20 or 30, well, that is the bathoom trip after a big eating binge, so that is not ideal and you want to act quickly if you see that.
But anyway, why would you use a poisonous substance to deal with a problem that can basically be dealt with with a flick of the finger? Save the poisons for when you really need them. jmho