Get Kathy Love's book on corn snakes:
Corn Snakes : The Comprehensive Owner's Guide by Kathy & Bill Love, 2005
You can buy it from Kathy Love at cornutopia.com.
Every corn snake owner should own it.
The book has information on sexing corns.
There are also numerous threads on this forum about sexing snakes, so you can do a search.
The basic answer is that you can't be sure what a snake is until you are breeding. If it has hemipenes, it is a boy. If it lays eggs, it is a girl.
Probing is pretty accurate, most of the time.
Tail shape is unreliable, but can give you a pretty good idea.