[header] How to determine the sex of a fish [title] Look for a nuchal hump on male fish. [step] A nuchal hump is a small bump on the forehead of a fish. The presence of one can be taken as confirmation that a fish is male. 
As a result, look carefully to see if a specific fish has or doesn't have a nuchal hump. [substeps] Many species of fish won't have a nuchal hump, and if they do, it might not be apparent.