Sexing a squid--determining whether it is male or female--may be difficult, depending on the species of squid. While people with fishing experience offer several suggestions for finding the sex of a squid, the most definite way is to look for a hectocotylus, an appendage or arm that male squid use to provide sperm to females, according to Danna Staaf, a Stanford University graduate student specializing in squid and author of the blog Squid a Day on the Scientific Blogging website.
Determine whether or not the squid has a hectocotylus. (Tree of Life Web Project, linked in the Resources section, includes diagrams of what the hectocotylus looks like and where you might find it.)
Examine the squid's mantle, also known as the tube. Male squids tend to have longer, more slender mantles than females, whose mantles are shorter and wider, according to a Weekly Times Now website article by writer and fisherman Steve Cooper. This sex identification method works best if you have multiple squid to compare.
Study the squid's markings. Cooper says males have patterns of lines running from the mantle to the fins, while females sport dots across their backs and fins. Staaf, however, notes that because squid frequently change their colors and patterns, this method may not be universally accurate.