Hi chrisbassist
Even if your lfs stocks male gouramis, guppies, etc., the chances of them having a single sex of corydoras is small. This is especially true if you visit them shortly after they have got a new batch in. You might find, however, that the clerk cannot tell them appart so be prepared to pick them out yourself.
Let me show you a picture as an example. These are mature albino C. aeneus. The male is the cory on the top and the female is the one on the bottom.
The most obvious difference is the shape of the female. Compared to the male she will be rounded and plump looking while he will be more slender. With some species this is more noticeable than with others. There is also a difference in the size of their ventral fins. The female has larger, more fan shaped ones than the male, but when you are looking at them in a tank it's difficult to get a good look at them.
This is a generalization, of course, with some species being less obvious, especially if they are wild caught species that may all be out of condition due to poor diet since they have been caught, but as a rule it's a workable method of telling them apart. If you are buying a group, chances are good that you will get some of each if you at least try.
When I shop, (assuming the fish all look healthy) I pick out the biggest corys in the tank and ask them to put them in a specimen container. These will usually be females and looking down at them in the container it's possible to get a good idea of their size and shape. Then I try to pick out more slender ones. If there is a difference in shape when they are added to the container, I figure I've done the best I can. If not, I ask them to remove them and try again.
The ideal breeding arrangement for best fertility rates is at least two males to each female when they are spawning. I usually buy my groups one male to one female since the females probably won't all spawn at the same time. I figure this gives me the best chance of getting a mixed gender group when making my selection.
