Sexing fish can be a very difficult task. While some fishes can be easily sexed, Cuckoo Wrasse as a prime example, others are nearly impossible to distinguish between without cutting them open and examining their reproductive organs.
There are various methods for determining the sex of bass, but there is no surefire method for determining their sex simply by physical appearance alone.
You can, however:
Observe the fish during the spring. Spring is the breeding season, and mature female bass can be easily identified by their swollen or sunken abdomens. Their abdomens will either be swollen because they are full of eggs, or sunken because they have recently laid them.
Determine the sex of larger bass by examining the area around their urogenital opening. This is the opening on the rear underside of the fish. Females will have a circular scaleless area around their urogenital opening, while males’ scaleless area will be more oblong.
Massage the abdomen of the fish by moving your fingers from the middle of the abdomen back toward the urogenital vent using gentle pressure. If milt (fish sperm) is expelled, then the fish is a male. If no milt is expelled, then the fish is likely a female, although not with certainty.
With Flounder the process is a bit trickier:
First step is “candling” the fish, a non-invasive technique in which the fish is viewed above a bright light source which illuminates its internal organs. The reproductive organs are identified and the technician records the sex of the fish. The same fish is then passed on to yet another technician who creates a small incision at the location of the sexual organs to confirm the sex recorded during the candling process. The otolith (or ear stone) of each fish, as well as a few of its scales, are removed and catalogued to later determine the age of the specimen. No way to do it on a beach or without proper equipment.
I leave it here, because if you are detirmined to find out the gender of a certain species you might resort to the powers of the interweb yourself. Hope I answered at least 2 questions.
Protocol for sexing Sea Bass:
http://om.ciheam.org/om/pdf/b63/00800919.pdfa good website for 'How to Tell If It's a Male or a Female Trout':
http://www.ehow.com/how_5852641_tell-it ... trout.html and it covers many more species.