The fisherman with an untrained eye might have trouble distinguishing whether a mature trout is female or male, but each has a few identifying characteristics. Those features, though, don't show up until sexual maturity. The sex of a juvenile trout is nearly impossible to determine just by looking.
Step 1
Check the calendar. If it is spawning season for trout in your region, you will have more luck determining the gender of the trout. Different species of trout spawn at different times of the year, usually in the fall. Check with your local fishery management agency to find out when the spawning season is for local trout species.
Step 2
Examine the trout's head. According to the Pennsylvania Fish and Boat Commission, a male trout will usually develop a hooked snout, or "kype," on its lower jaw. Male trout are usually more elongated, too. The female trout's mouth is more rounded and is smaller than the male's. Her body also tends to be more rounded.
Step 3
Look at the color of the trout during spawning season. The male will have brighter colors than the female.
Step 4
Check the abdomen of the fish and determine if it's distended. If so, it more likely will be a female carrying eggs. Note that a trout that just ate could show a similar trait.