You know you've done an OK job when you see some of your students after the ACT, you ask them how it went, and they say with a smile: "Not bad!"
Then you ask them about the essay, and they say, "It was hard!" So you realize that maybe you didn't do such a great job after all.
(I was actually bummed when they told me the persuasive essay prompt had something to do with requiring students to take classes on saving the environment or something like that. Out of all the prompts I created, I missed the obvious question about GOING GREEN. Darn, that was supposed to be in my lesson plan!)