Male and female pinnipeds (seals, sealions & walrus) are often sexually dimorphic (the two sexes look different). Leopard seal females (♀), when fully grown can be substantially larger than males (♂).
Regardless of the size or age of a leopard seal, a clear view (or photograph) of the ventral surface allows the sexes to be differentiated. Both sexes have a scar from the umbilical attachment (belly-button). The males have a penile opening approximately half way between the umbilcal scar and the hind flippers, whilst the females have two small mammary glands which are often visible.