Popping small colubrids like cal kings and corns takes the right touch. Just to see someone do it or even to be taught, does NOT mean you will get the sex right when you try.
Like Ken said, if you see the 'penes, it's a boy. If you don't, ????? But with the females, you will learn what to look for. You don't see hemipenes but they are a bit different. I always sex the "females" several times before I label their bin.
Females are always hard to sex...
Now, probing is a different story. THAT is the ideal method. But if the snake is too small to probe, you have two choices: to pop it or to sell it unsexed.