Sharks are sexually dimorphic.
That is, there are visual differences between males and females. Males, like the one seen in this picture, have pelvic claspers which are modified pelvic fins used for sperm delivery. Claspers are rolls of cartilage that become stiffened with calcium in the adult. The presence or absence of these claspers make it very easy to differentiate males from females. Males also have paired testes, however the right one is always more developed than the left, which may be smaller or absent altogether.