Try looking down at them from the top of the tank. I find it easier to judge their build that way because even if they are moving you are able to see their shapes clearly.
If your corys are not yet mature, it will be difficult, and often impossible, to tell the sexes apart. Once they are mature, the difference will be clear if you have both sexes in your tank.
As Inchworm said, Corys are usually dimorphic in body shape, but some show differences in the shape of the ventral fins. Males have spear-shaped fins, whereas females have rounded fins.