• Two mirrors.
• Take a photo or video holding your smartphone behind your head pointed at you.
• I suppose if you needed to see it in roughly realtime, you could figure some way to live-stream your smartphone camera to a screen in front of you.

The only other ideas that are coming to me are joke answers like “bend over backwards and lay the back of your head on the bed of a photocopier” or “visit a hall of mirrors.”

What’s your purpose for doing so?