The males tend to have a fatter, rounder mellon, while the females are more sleek and streamlined. There are various ways a dolphin has of showing that she or he is interested in sex. Males are probably the easiest to detect. If you are in the water, they may rub it along any part of your body, or wrap it around your wrist or ankle. Their belly will also be pinkish in colour, which also denotes sexual excitement.
Females can be a little harder.
The most obvious way a female dolphin has of displaying her sexual interest is the pink-belly effect. Their genitals become very pink and swollen, making the genital region very prominent. They may be restless, or they may be acting as normal. If you are out of the water, they may swim up to you and roll belly up, exposing themselves to you, coupled with pelvic thrusts. If you are in the water, they may press their genitals up against yours, nibble your fingers, nuzzle your crotch, or do pelvic thrusts against you.