Yup minghan got some of the facts right. Some specimens do exhibit sexual differences like C.Undulatus. SOme in colors while others in finnage and so on.
Go to yahoo and search for corycats.

It's Ian Fuller's site

Great info in ID of cory and how to identify them. Even better if you can get your hands on his books on breeding corydora cats. Of course he will tell u in more detail in his book on how to ide males and females.