I cut my hair with a mirror they sell online. It's 3 mirrors and it can hang from a door. You adjust the mirrors on the side until you can see most of the back, turn your head and look thru the other mirror to get the rest
Two mirrors. Or 1 mirror and your phone’s camera. Or even just the camera if you don’t care about seeing it in real time. Or go the extra mile and have fun with it by building a telescope that encircles the earth.
I believe if you gently step over the event horizon of a black hole, the light reflecting off the back of your head will wrap around accretion disk and back into your eyeballs. It requires a bit of setup and is kind of a one-time thing, but you'll get a much better view than you could get with a mirror!
This didn’t work for me at first, but I tried it a second time and it actually worked! You weren’t kidding about it being a one-time thing though, because I haven’t been able to do it since.
Small correction: You'll want to be at the photon sphere, not the event horizon (1.5x the Schwarzschild radius). You can theoretically hover there for an unlimited time, and even escape, as long as you don't mind the immense acceleration necessary. :)
Easiest way is to stand with your back to a large bathroom mirror or vanity while holding up a hand mirror to your eyes to look back at the big mirror.
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.”
Get a handheld mirror, and use it in conjunction with your bathroom mirror. Or use your cell phone to take a picture of the back of your head, using a timer if it’s too awkward to hit the button.