In general, male corys are smaller and more streamlined, while females are larger and more bulky/fat- you may have to wait a little while though until the sex's are obvious if your new corys are quite young and small and not very mature yet- doing a search yourself though on these topics as smithrc suggested is priceless .
Female corys are usually a lot bigger than the males. Here's a picture of some adult black C. aeneus so you can see the difference.
While it's much harder to tell the difference between them with some species than others, I saw on another thread that you have C. aeneus and C. paleatus (an albino and a peppered cory), and they are not that hard unless they are very young. If you can take a picture from above, like the one I posted, I'll see if I can tell you what you have.
I have had my 6 group of bronze corys for three days now, but they are rather small and im guessing they haven't reached maturity yet. Hopefully I will get 3-4 females, if not once they mature I may have to trade some for different sexes.
Your corys will probably not get to 3" in length, few albino C. aeneus do, and not even all peppered cory females (C. paleatus) do, but they will still grow larger than they are now.