Sex determination in cephalopod?
So far, you're right in that there is no hard method of determining cephalopod sex determiners.  Male and female share the same number of chromosomes, but are morphologically different, with the male developing a special arm for transferring spermitangia and the females often being larger than males (1). 

Most research papers seem to focus on just being able to tell the sexes apart by behavior/anatomy (2) and have not yet broached on the How, except to note that they do not seem to have specific markers, which you've already noticed (1).

You can tentatively run with this, and keep an eye out for new research in Google Scholar, and decide that your race uses environmental signaling (3) to determine their sex.  Given how real life nature goes, you'd probably be in the clear as certainly one cephalopod species will use that method to determine sex. It also can segue into sequential hermaphrodism if you're interested in going in that direction.  It would certainly bring a novel element to your sophonts, rather than making an octopus "human". (Hypothetical extrapolation based on the previous version of this question in Worldbuilding.stackexchange)

But, to give you some ideas for environmental factors.  Some species of Daphnia will make parthenogenetic males when three factors meet: a short photoperiod, shortage of food and a high population density (4). The ever popular crocodiles have temperature dependent sex-determiners, with a narrow "sweet spot" to get a 50/50 male/female clutch (5). The green spoonworm determines it's sex as a larvae, if it lands on the seafloor it becomes female, if it lands on a female it becomes male and lives as a parasite inside of her. 

Until further research, develop some form of environmental conditions that can be met naturally, or generated by intelligent octopi for what you want this question to answer.

(1) Coe, W. R. (1944). Sexual differentiation in mollusks. II. Gastropods, amphineurans, scaphopods, and cephalopods. The Quarterly Review of Biology, 19(2), 85-97.

(2) Mercer, M. C., Misra, R. K., &amp; Hurley, G. V. (1980). Sex determination of the ommastrephid squid Illex illecebrosus using beak morphometries. Canadian Journal of Fisheries and Aquatic Sciences, 37(2), 283-286.

(3) Rider, C. V., Gorr, T. A., Olmstead, A. W., Wasilak, B. A., &amp; LeBlanc, G. A. (2005). Stress signaling: coregulation of hemoglobin and male sex determination through a terpenoid signaling pathway in a crustacean. Journal of experimental biology, 208(1), 15-23.

(4)Kato, Yasuhiko; Kobayashi, Kaoru; Watanabe, Hajime; Iguchi, Taisen (2011). "Environmental Sex Determination in the Branchiopod Crustacean Daphnia magna: Deep Conservation of a Doublesex Gene in the Sex-Determining Pathway". PLoS Genetics. 7 (3): 1–12. doi:10.1371/journal.pgen.1001345. PMC 3063754 Freely accessible. PMID 21455482.

(5) Janzen, Fredric J.; Krenz, James G. (2004). "Phylogenetics: which was first, TSD or GSD?". In Nicole Valenzuela; Valentine A. Lance. Temperature Dependent Sex Determination in Vertebrates (PDF). Smithsonian Institution. pp. 121–130. ISBN 978-1-58834-203-4.