I had a strange dream this morning.
I dreamed that I was in bed asleep when my clock radio woke me to the sound of a symphony playing on my customary classical music station. Time to get up. I threw off the covers and swung my legs around to sit on the edge of the bed. Leaning over, I tapped the the clock radio's off button. The music kept playing. I tapped again, harder. The music kept playing. Puzzled, I turned the knob to change the radio station. The symphony continued undisturbed despite my actions. I felt perplexed and alarmed.
Then I woke from the dream. The clock radio was indeed playing a symphony, the same symphony that had somehow seeped into my dream. But now when I hit the off button, the music stopped.
This is how you know you are truly awake: your actions produce expected results.