Some people don't really know how to do this, but when you swallow, a 'flap' covers part of your throat so you don't inhale food/water. If you close that 'flap' and try to inhale, it won't work. If you do it in a dream, you'll still breathe. I always use that as a reality check; it's really inconspicuous.
I forget nearly half the time going through doorways, though. This one really is hard to start up... I should get a watch implanted in my arm or something that shocks me every 30 minutes so I do regular reality checks.
