Sex change without surgery occurs in a number of animal species. It is known as sequential hermaphroditism. Some animals are hermaphroditic and have both male and female reproductive parts and others switch between the being male and female. Usually, this switch goes one way and is not reversed, such a strategy is known as sequential hermaphroditism. I see a lot of these sequential hermaphrodites on the reef. Some of the most colorful fish on the reef are parrotfish (see Fishes in the Sea Part 2 and 3). Generally the most colorful parrotfish that you're likely to see, like this stoplight parrotfish, are male, but most of those started out as females or as males that are incognito. Here's a picture of a large female. The typical sexual cycle for one of these fish is to start as a little juvenile female and at some point enter a harem and mate with the large, colorful, and desireable male controlling the harem. At some point, the big male is going to lose control of his harem because it gets to big and splits or he dies and at that point testosterone in the largest female will skyrocket and turn the female into an aggressive, harem guarding and colorful male. At this point the fish is said to be in its terminal phase. Turning male is the beginning of the end for these fish and the last sex that they'll be. At this point in its life, it will spend considerable time time fending off other males, including those that look female.
So if you see a pretty parrotfish that looks like it has makeup on its face, it's a male.