How Do You Know When You're Ready For The AP Chem Exam?

I would say you know you're ready for the exam when you get a 5 or a 4 on 2 practice exams in a row in any of the prep books.
Getting a good score once might be a fluke, but getting a good score twice shows that you're ready for the exam and that you truly know what you're doing.
You could also look through all of the questions on the exam and try to figure out within 15 seconds if you know how to do them or if you know how you would start doing the problem. If you can figure out the process of solving it within 15 seconds then you're probably ready for the actual thing!
Either way, the AP chem exam usually isn't that bad and I heard that the curve is pretty generous so there's that also!