With an assistant holding the snake’s body (with the belly facing up), a probe is inserted under the large cloacal scute (scale) in a direction pointing towards the tail. Bending the tail backwards slightly makes it easier to find the cloaca. Use the probe to gently poke around (while slowly twirling it to facilitate forward movement) to find either opening that’s just off-center in the tail’s base. Lubricant such as K-Y Jelly, or even just plain water, may be applied to the probe first to make it easier to insert. Angle the probe a little in various directions while probing until an opening is found, then try to slide it further toward the tail tip. Only the very slightest pressure is used when looking for the hemipenes’ openings. Too much pressure may puncture body tissue and cause injury.

Because of the elasticity of the hemipenes, the probe will have a slightly ‘bouncy’ feel if fully inserted into a male's organ. The same procedure on a female will produce a much firmer ‘dead stop’ as the probe encounters only a muscular wall at the base of her tail. When king snakes are probed, it is common to encounter a few drops of blood and is no cause for concern.

The picture below shows the ‘belly-up’ views of the tail regions of a female (top) and male (bottom), including an x-ray view of where the hemipenes of a male is situated. Females have small pockets / scent glands in their tails, but they’re never nearly as deep as the hemipenes of males.

If the snake is a male, the probe will slide down into one or the other hemipenis to a depth more than the width of the tail base. On a baby corn snake, for instance, this depth would be about 3/8 – 1/2 of an inch. It will barely penetrate (only 1/16 – 1/8 of an inch) into the scent glands on a baby female corn snake. An adult male corn snake that is 3 feet or longer will have a hemipenes that probes 2 – 3 inches deep. By comparison, an adult female corn snake will probe only about 1/4-inch deep.

Some sets of probes have wider ball tips as shown in the diagram, and others are simply smooth, slender rods with rounded tips, both types are precision made instruments which are safe to use on Corn snakes, however, great care should be taken to use an appropriate size probe for the snake, and no probe with a tip greater than 4mm should be used, even on the largest of Corn snakes.

The safest way to sex a baby Corn snake is by 'Popping'

It's called 'popping' because you literally 'pop' the hemipenes out of a male snake, a female will show little more than two tiny red dots instead of hemipenes. From an experienced herper, popping should be around 80% correct. But popping together with probing, should be at least 95% correct.
Remember, both of these methods should be done with experienced herpers, inexperienced herpers doing this might have the possibility of injuring the snake and making future breeding not possible.

If you feel confident about 'popping' your own baby snake, below are some photo's of
my husband popping one of our baby Corn snakes along with some instructions.
"It's a boy"  LOL

First, hold your baby snake with it's vent (cloaca) facing you.

Then slide your fingers holding the body end of the snake, closer to the vent.
Support the snake by it's sides, gently, but firm enough to hold the snake in position.

With your other hand, place your index finger under the snakes vent, and place
your thumb on the snakes tail with the tip of your thumb about 6mm (¼inch)
from the vent, then apply a little pressure with the ball of your thumb, thus
lifting your thumb tip off the tail and making the tail bend downward slightly.

Now 'roll' your thumb (DON'T SLIDE) towards the vent. When your thumb tip
gets close to the vent, one or both hemipenes will 'pop' out if you have a male snake.

This picture shows the hemipenes fully out. If nothing pops out, try
again, but move your 'popping' thumb slightly further from the vent.
 If you are 'popping' a female snake, you should see two tiny
red dots appear in place of the males hemipenes.

The male hemipenes in
a little more detail !!

Two WARNINGS!
1. Snakes reproductive organs can be damaged by applying too much pressure while popping.
It is always best to 'pop' a known male first, so as to perfect the procedure.
2. Baby snakes often 'poop' on you when attempting to 'pop' them !!

Good luck & be VERY careful.