Females are usually a little bigger and they are fatter in the belly that males. Males are much slimmer. The best way to see this is to look at them from above.
I've noticed that the back set of fins underneath the cory (pardon my anatomy ignorance) on the female are more paddle shaped than the males which are kind of pointed like the blade of a knife. Hope this is helpful.
Out of 11 cories I've recently looked carefully at them and realise that I only have one male No wonder I never get any eggs off them. My peppered cories spawned a couple of times and there is one male amongst them so the poor guy was worked pretty hard with 3 females to please Have been to the lfs today and managed to get a male spotted cory but they didn't have any others today so will be on the look out for some more males next week.