The females are larger, and 'plumper' than the males.
That means that when seen from the side, the males will have a flat stomach, whereas the females will have a rounder stomach. It takes a little getting used to, but isn't too tough when you get used to it.
Is there any other way to tell? I've heard of a few things like One will be longer than the other, but I can't remember which one it was, or that one's ventral (I think) fins were more pointed than the others.
This is from Ian Fuller, aka "Coryman" - expert corydoras scientist. Basically, if he says it, its the current understanding. If he doesn't say it, then it just isn't so.