They are easy to sex. Besides the mature female being significantly larger and fatter, the mature male has a gonopodium. This tube-shaped fin acts like a penis does in mammals. The female also has a black patch near her vent where fry develop. She will greatly increase in size as the fry develop.
Look for a "gravid spot" at the rear of the curve of the belly. It will be a darker spot, right above the fish's anal fin. If it has a gravid spot, the fish is a female. This is the easiest way to sex guppies at a young age and I believe it's the same for mosquito fish.
