Housing multiple corns together involves some health risks for the snakes. There have been numerous threads on the subject, and you might do a search to read up on different peoples opinions. Basically the issues with keeping them together include:
1. The inability to sort out who did what (shed, poop, etc.)
2. The risk of accidental breeding at too young an age and egg binding
3. Spread of disease
4. Risk of Canibalism
While you might find that you can house them together and not run into any of these problems, most people in this forum take a cautious approach and prefer not to expose their corn snakes to any of these risks that can be avoided by simply keeping them separated.
In answer to your original question, the sex of the hatchling can be determined by a method called popping, in which the tail is rolled between the thumb and finger to "pop" out the hemipenes of the male snake. Since this is a rather difficult procedure to master, it is not recommended for beginners.
Mark