A. This question is more ichthyology than stream ecology, but maybe I can help.

I'm assuming that you're interested in how to determine the sex of a trout externally, rather than internally. The latter is quite simple; you merely have to identify either the testes or ovaries.

External identification of sex can be trickier, depending on the age of the fish. Here are some things to look for:

Morphology: The shape of the vent, or anus, which is just in front of the anal fin, is useful in distinguishing the sex. In females, the vent is round, whereas in males it is triangular shaped. This characteristic is about the only way to sex smaller fish. As trout get larger, other morphological characters become useful. In males, particularly brown trout, the lower jaw can become extended and develop a "hook" shape (called a "kype"). This characteristic is often manifested during spawning in many salmon. Females retain the rounded head shape. There is another morphological characteristic that can be used for brown trout. Upon becoming sexually mature, the male brown trout develops a convex anal fin, whereas the female retains the concave, or falcate, fin shape.

Coloration: In some species of trout (especially cutthroat and brook trout), males become brightly colored during spawning season.

Hope this helps.