I agree that it's hard to gauge (for me, anyway). Lately, I've been trying to take advantage of Paraliminal sessions. As I get deeply into a Paraliminal tape, I try to set the same anchor each time (NLP term). I don't seem to get as deep when I fire the anchor but I can tell a difference. Additional sessions should strengthen and deepen the anchor.

If you aren't sure about anchoring, it's a method for "bookmarking" a particular state. When you are in that state, find an action (or internal image, or internal sound, whatever works for you) and perform it when the state is at its most intense. For instance, when you are feeling particularly successful at achieving the accelerated learning state, you might pull lightly on your lower lip. If done correctly, pulling on your lower lip in the same way in the future should help you reclaim that state.