Male and female orca can be distinguished by markings in their genital area. Females have mammary slits (dark spots) located either side of the genital slit (which also typically has a dark colouring). Male orca do not have mammary slits and their genital slits tend to have a more pronounced dark mark. The white patch surrounding the genital slit of male orca also tends to be more elongated.
Thanks to Bart Rulon of Island Adventures Whale Watching for this amazing photo!
To find out more about identifying Southern resident orca, please visit: http://www.whaleresearch.com/orca_ID.html