As it goes with most skinks, sexing ocellated skinks visually is near impossible. Despite my best efforts to find any helpful tidbit regarding ocellated skink sexing techniques, the studies surveyed which mentioned sexing the subjects used invasive procedures to do so. Probing for hemipenes has proved unsuccessful for this species (Graves & Halpern, 1990).
Some say that females are significantly larger than males, but speaking from my own experience and observations, size seems to have more to do with an individual’s rank in the social hierarchy than sex. Females do seem to be more likely than males to become exceptionally large, but not all females will do so. Males do seem have consistently wider heads (Taylor et al., 2012) relative to the neck (more jowl-y), but this is not always easily identifiable.
If you want to house a group of unproven individuals together, you’re going to have to wait and very carefully observe the interactions between individuals in order to determine which ones are males and which ones are females. Males are likely to bite and wrestle each other in competition for females (Rugiero, 1997), and they will chase and mount sexually mature females. Females will nip at each other and chase each other, but they do not mount, and very rarely wrestle (if at all).
According to Reproduction in the Ocellated Skink, Chalcides ocellatus (Squamata: Scincidae) in Israel, already established in Mesa, AZ published by S. R. Goldberg in 2014, ocellated skinks from Israel mature around 68mm SVL* and females around 75mm SVL. In Cicek & Göçmen’s 2013 paper, the rate of gonad development suggests that ocellated skinks can be considered adults after attaining 55mm SVL. Breeder Jeff Littlejohn reports from his experience that ocellated skinks take 2 years to mature.
In my experience, ocellated skinks are capable of reaching sexual maturity as young as 1 year old, and groups of juveniles should be separated accordingly to prevent undesired offspring and/or inbreeding.
*SVL = snout to vent length