A. I am assuming that you are asking about how to distinguish the sexes by external characteristics, rather than the obvious internal differences. What you are asking about is called "sexual dimorphism," or how to males differ from females. Here are a few external characteristics that may help. These pertain to all trout species unless specifically indicated.

Vent: The vent of male trout is usually slender and almost triangular in shape, whereas that of females is more circular and often slightly protruding. Both of these characteristics become more pronounced and noticeable as the fish matures and grows larger.

Jaws: As males grow larger, they develop a hooked lower jaw (called a "kype"). This is especially noticeable on brown trout and least on brook trout. Obviously, this characteristic won't help you much on smaller trout.

Coloration: This can be tricky, especially if you are around streams where anadromous trout live. Here, fish coming in from the ocean are generally silvery, but they lose this brightness the longer they are in freshwater. In general, the males of most trout species develop brighter and more intense colors during times of spawning. However, this doesn't help much unless you have another specimen for comparison, especially a known female.

Fins: It has been noted that the trailing edge of the anal fin of sexually mature males brown trout is convex in shape, whereas that of the sexually mature female is concave or falcate in shape.