There are supposed differences but I have never been able to see any.
If you want to try breeding, get yourself a group of at least six and you are virtually guaranteed a few of each sex. Obviously, the larger the group, the greater the odds
In my experience of Peppered Corys, the females appear to get quite substantially larger. I'd say almost twice as big. That's probably the easiest way to tell, but again won't work on juveniles.