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Most octopus are sexed the same why, which is more or less a guess. You can tell by the third arm on the right. On a female the suckers well be uniform in size with larger at the base and smaller at the tip. On males there will be some suckers that are larger than the surrounding ones and some times the tips of the arm will be slightly modified. They usually have to be adults to be able to sex them, mainly because it is easier to tell on larger animals.
Enlarged suckers on the side arms are certainly an easy way to identify males, but it's true- they don't develop until they males are mature. In some species very large females have them too. In others, neither sex has them. A sure-fire way to ID a male is to find the hectocotylus (the third right arm). There's a small diagram in this thread (scroll down) that may help identify what to look for.