Two mirrors
Back to the bathroom mirror, hand mirror in front of your face. Move slowly: everything is reversed twice, so it takes a few minutes to get used to.
Light from above and behind
Shadows hide missed patches. A bright lamp angled at the back of your head makes uneven spots obvious.
Your phone camera
Film the back of your head in video mode and watch the playback, or prop the phone up and use the front camera. A bigger screen helps even more.
Short strokes, frequent checks
When you can't see, go slowly, use short strokes, and check by touch as well as by view. Ask a barber or friend to set the first clean line, then maintain it yourself.
General grooming tips, not medical advice. If you have a skin condition, a mole that changes, or irritation that doesn’t settle, see a doctor or pharmacist.