|>>|| No. 6275
Buy a second mirror you can use to check the back of your head (or use the camera view on your apple watch to scout out the area) and you'll be fine. As others have mentioned, pay attention to the area around your ears (behind the ears in particular), if using clippers there's really no harm in going over an area several times to make sure. Feel your scalp "against the grain", you'll need to figure out which way your hair grows, and you can find bits you clearly missed.
Your hair generally grows forwards on top of your head, downwards at the back and sides of your head and then you get the really nasty worly area where those directions meet. The large bits that grow in one direction are easy, the borders are what you need to pay the most attention to. With clippers you'll get nothing going one way, then change your angle slightly and you get a whole carpet's worth. Rub in circles to find what you missed.
If all else fails, you can use your phone to film a 360 view of your head, using as many angles as you can, to find missing spots. With a bit of practice, though, you'll know your problem areas and automatically pay extra attention.