zìjǐ kàn bù dào hòumiàn, kěyǐ qǐng lǐfàshī yòng xiǎo jìngzi gěi nǐ kàn

自己看不到后面，可以请理发师用小镜子给你看。

If you can't see the back, you can ask the barber to show you with a small mirror.