The male red arrowfish has an elongated body, and the anal fin near the anus is a long, thin, rod-like copula.

Female red arrowfish are short and fat, and the anal fin near the anus is an open fin like the common fish.

Red arrows belong to the medaka family of tropical fish, and as such are ovoviviparous, which means that the eggs hatch inside the female and give birth directly to small fish. So the males need to insert the mating stage that the anal fins have evolved into to fertilize the females, so it's easy to tell by the difference in body size as well as the difference in genital shape.