I've only had 2 (short) LDs so far but both times I could tell when it was about to end. The dreams went from crisp and clear to slightly blurry and distorted like looking at the reflection in a perfectly still body of water and suddenly the wind starts blowing and brings everything into motion. In the most recent LD I was able to successfully stabilize the dream twice by rubbing my hands together when the distortion started. When the dreams ended I woke up immediately. Being still a newbie with all this lucid dreaming stuff I blame it on my excessive excitement I remember both dreams as though I experienced them in waking life. Will be interesting to see how this develops as I continue down the road.