You can't easily tell them apart. It's too difficult, they're too much alike and won't hold still. So, do what we all do, just choose a name and choose a sex. You can have a look at what people have named their octopuses in the List of Our Octopuses thread at the top of Journals and Photos.
"Although gender is often used interchangably with sex, today many do not think that sex and gender are the same thing. "
So much for connotation and denotation.
My octo is named Trapper after the occupation of the person who caught it. She is at the end of her brooding cycle (and life unfortunately) but we did not change her name when the discovery of her actual SEX was established