If the corys are young, it's often difficult to tell them apart. Once they begin to mature it becomes easier.
The best way to look at them is from the top and compare them to each other. The females will be longer and wider than the males. Also, the female has larger and more rounder ventral fins than the male who has small. sort of pointed ones.
I don't see any corys in your signature. Are you thinking of getting some?