It's called vent sexing and I believe can only be done on chicks no older then a couple of days, and if done incorrectly can injure the chick.
So ideally get it taught to you in person.
I'm still slightly inclined to pullet, as my BO cockerel at that age was very obvious but could just be a slow developing cockerel.
Do please keep updating, it's very interesting, and useful for me!