Usually with adult King Snakes it is pretty easy to tell visually, especially if you have representatives of both sexes to compare. The male's tail will be noticeably longer than the female's and the taper will be more drastic in the female.
Popping is usually only done with neonates. If you can't tell visually with an adult, the only reliable method (short of going to a vet and having it x-rayed) is by probing it with stainless steel sexing probes. This needs to be done by an experienced person, because it is very possible to injusre the snake if it is done improperly.