You are working on an undercover spy mission, equipped with an iPhone to communicate with your partner. Assigned as the lookout, your job is to make sure that no nosy guards pass by and to alert your partner (through text! :D) if anyone does.
As the night drags on, you feel drowsy and for a brief moment, your phone slips from your hand. Startled, but unable to look away from your binoculars, you fumble around for your phone and eventually grab it.
However, after the brief tumble, there is no way of telling if the screen of the phone is on or off. Should it be on, the light would blow your cover, risking not only the mission but also your partner's life.
Without looking at the screen, what is a sequence of button presses that can guarantee the iPhone is off?