Sexing Corydoras Catfish

Aspidoras, Corydoras, & Scleromystax are best bred in trios, one female to two males. 
Corydoras
Unlike lots of other fish, this genus does not usually show clear secondary Gender dimorphism, visual indicators of gender, such a colour differences. However one method of determining gender seem universal with these fish, body shape when viewed from the top. Being heavily plated, females cannot expand like many other fish to accommodate eggs. Her body shape seems to be fixed from this purpose from a young age, making it easy to pick out females fairly quickly. The image below shows the differences between males & females.
Males have the widest part of their body next to the Pectoral Fins, just behind the gill. From there, the body follows a straightish line to the caudal peduncle.
Females, on the other hand, have the widest part of their body round about halfway along and are more curved towards the caudal peduncle. Once you get your eye-in it is possible to spot females from the side also. Even little Corys, such as C. pygmeaus, show obvious rounding in the middle from as little as 15mm long.
Pelvic Fins also differ to some degree or other but are hard to see under normal conditions. Males can have more pointed fins as in the illustration. Females have rounded fins, which are used to hold two or more egg when they look for a place to deposit them. Both pectoral fins look like a pair of hands cupping the eggs during the search for a suitable spot.
Size also differs, females growing slightly bigger, this stars showing up in fish less than half grown.
Other indicators are elongated Dorsal fins, elongated Pectoral Fins, colour differences, but these vary between species and are mostly not present.
Scleromystax
These are much easier to differentiate because males can be more brightly coloured, have bristles on their cheeks, and have elongated Dorsal & Pectoral Fins.
Aspidoras.
Follow the suggestions for Corydoras