There is another possible way to sex the Corydoras however I'm not sure it will work for every species. Also you need good eyes, corporative Corydoras and lots of patience and observation from you to do that.
But here is how.
I heard/read(don't remember where though) that the shape of their vental fins are slightly different from males and females. The females have more fan shaped vental fin and males have more pointed shape.
And I can see that in my Pandas and Sterbais(although they haven't layed eggs yet but I see the one courting and chasing have pointy vental fins so I assume they are the males)
And I do not own other kind so I'm not sure this would work for every species.
And this method would be really tough just looking them at the dealer tank. Unless there are only few of them in the tank or put each one to the specieman container or tank to look closely.
And of course, the bigger the fish, bigger the fins and easier to tell them apart. and by then, usually body shape is difined by then.
I hope this would work for you. But for me, I usually get good number group of 6 or more when I get the small juvi and just roll the dice that there are both sexes in the group. Unless you know the store and personal well and they accomodate you, it is almost impossible to pick the sex of young Corys from the store.