How do we know whether when we dream we are someones waking and when we are awake we are someones dream?

Poets and philosophers have posed similar questions throughout history. However, we now have the ability to observe the activity of the mind through brain scans, as well as many years of documented research by psychiatrists, psychologists and therapist. While there continue to be mysteries surrounding both dreaming and the mind, we can be quite certain that all dreams take place within the mind, and the brain, of the dreamer.