Many things....nuchal hump, dorsal fin length, facial markings. The longer the fins and larger the hump the better chance of a male. However, if you can get him to cough that would be ur best bet.
black Dorsal fin spot means theres a good chance its female, body shape alot of times females have a more (american)football shape than males, males generally have larger nuchal humps (kok) but I have seen females with massive heads and males with no kok at all.
the most accurate way is venting.
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