How can we realize that we are dreaming when in a dream?

I haven't been able to do this until lately. In this dream, I was free climbing with my Dad. Suddenly, a flying Tony the Tiger came out of nowhere and grabbed my ankles. He was pulling me off the rock face! I suddenly remembered that anything is possible with Jesus Christ. I started to spin my feet around like a propeller. Tony let go and flew away. The dream was over. In a way, I realized it was a dream because I did something otherwise impossible.