Actually, people often probe their hatchlings right after their first shed. Females will probe somewhere around 2-5 sub caudle scales deep and males will probe around 10 or so. There's a noticeable difference.
Popping can be done just as early as probing but it is a bit less accurate when being attempted by a beginner. If you pop the snake and the hemipenes show up, you've got a male. If no hemipenes show up, either it's a girl OR you have a male and you just could get them to “pop out”.
Tail length and shape can be used to sex corns but it takes a bit of experience and I wouldn't rely on it if you have any breeding plans for the snakes in question. It really sucks to go all the way to the breeding point only to find out one of the snakes was sexed wrong.... trust me!