It's quite difficult to tell on a bimac. As a result, each of our bimac owners just decided on one sex or another and chose a name. I thought that Ollie was a male until I saw her with a string of eggs! Same was true for Carol's Ink. And it looks like Roxy, who was given a female name, is actually a male.
I have never managed to visually sex an octopus while it is still alive! You may get lucky and spot the groove running along an arm called the hectocotylus which is a modified arm the male uses for passing sperm to a female.
In that case you have a male, if you dont see it, it may be a female or a male whose hectocotylus you cant see!!! :)
I forget where I found the information, but when I first got my bimac, Andy, I read that for this particular species, bimaculatus, males do not possess the modified tentacle or hectocotylus, but instead have obviously enlarged suckers on the 2,3 and 6,7 arms. This feature is only noticeable in mature animals. Species where there is a hectocotylus, it is usually on the third arm. The pacific octopus at the Long Beach Aquarium is very easy to sex as a male when he comes out because of this feature.
The arms are numbered in pairs (1st left arm, 1st right arm, etc). Pair 1 is the pair of arms that is most dorsally located, i.e. the furthest away from the funnel (the 'front' pair in an octopus). The ventral pair, nearest the funnel, is pair 4 (at the 'back').