I know I'm falling asleep when the movie of thoughts and images playing in my head becomes more like watching something play out on its own than thinking it myself. The thoughts and scenarios stemming from myself become more vague and start to come from somewhere else, basically transitioning from inner-monologue narration of images to compiled dreams that I no longer have control of. However, if I think about my own falling asleep as it's happening the self awareness wakes me up.