Of all the corys I have or have seen, I think the bronze C. aeneus is still my favorite! While they may seem plain compared to some that have more elaborate markings, they more than make up for it in playfulness and personality.
When they are young, they have the most lovely coloring, too. They are almost iridescent when you see them in the right light.
If you have both male and female corys, the females will be the larger ones. If they are all the same size, then they are probably either one or the other. Males will be a little smaller and less "chunky" than females.
Another way to tell is by looking at their pectoral fins. A female's will be rounded and larger than the male's. She uses them when they are spawning to carry the eggs an deposit them on the glass or plants. The male's pectoral fins are smaller and more pointed.