As a guide, I am often amazed at the look of someone's face when I comment that the fish he or she has just caught is a male or a female. "How can I tell?" they immediately ask while examining the underside of the fish for an obvious visual cue. Below are a couple of images that explain the difference. Note, it is often more difficult to determine a trout's gender when it is still a juvenile, ie under 9 or so inches.