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female or male

3rd right arm. If it's the end is the same as the rest, female. If it's got a different end (no suckers, almost flat or blunt), it's called a hectocotylus, and the octo is a male.
It is hard to be sure until you see eggs or not, but males have a Hectocotylus, which is what they use to transfer sperm to a female. It is typically the 3rd right arm of the octo. The octopus will usually keep the end of his Hectocotylus coiled at all times to protect the tip which is where the opening to transfer sperm is. There are other features that could distinguish it (depends on species the octo could have any or none of these) that include larger suckers and that arm being a little shorter than the others.

Edit: Grrr... Animal Mother beat me to it

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