I am currently unsure on whether or not I am still mouth breathing in my sleep, and never am able to tell in the morning whether or not my oral posture is good, as my attention is fixed on getting a cup of coffee when I awaken.
I've always had trouble breathing out of my right nostril, and barely feel any airflow coming from it when I sleep. This side of my face seems to be very slightly dystrophic compared to the left. I'm thinking that this is somewhat in part due to the dominance in breathing by my left nostril and little to no airflow in my right.
There is also crowding around my right canine, which could be due to the underdevelopment of the upper jaw in that region from not needing to grow because of the left side doing all the work. Now that I have it covered, there is a warm feeling in the whole of my right cheekbone/zygos. Hopefully I can get some bone reformation at this point in time being 20yrs of age (previously posted as Jkizzle but don't like that username).
Maybe after I can stop going from sleeping on my back to the side while I sleep I'll know for sure whether or not I'm keeping my mouth closed during sleep by whether or not there's tape still covering my lips.
Crazy as it sounds, I have actually seen some myofunctional therapists recommend mouth taping during sleep. I've never tried it because I figure, even with the lips taped together, the tongue can still drop and the teeth can still part, but I guess at least you would know that you are breathing through your nose. If you want to try it, I suggest using a tape that is safe for your skin. There is a brand called Snorless Strips that is specifically intended for a practice called Buteyko Breathing. You might test it out while you are awake and see how it feels. I guess some people put a piece of tape all the way over their mouth while others just put a smaller piece across the center part of their lips to hold them together.
To add to this, it might be beneficial to wear the tape just before you go to bed, to let your body get used to it. I end up with the tape off my lips, but signs that I kept my mouth closed, when I don't do this. The 3 times I've put it no 45-30 mins before bed, I woke up with the tape still there. The main thing I have difficulty with is not moving on to my side while I sleep, but I've been getting better about it by keeping my head and neck straight as I fall asleep and not tell myself that it's okay to have it deviate slightly to one side.