This is a bit subjective, but I'd consider being able to tell the difference between male and female skeletons a Knowledge (anatomy) check, or maaaaaybe a Heal check if I'm feeling generous (perhaps having 5 ranks in Heal would give a synergy bonus to the Knowledge check or something). Identifying the sex of a skeleton, especially one from a different species, seems like it would have more to do with Intelligence than Wisdom. Redcloak is smart, of course, but identifying the sex of a skeleton is an applied skill, not a wholly intuitive one, so I'm not sure his raw intelligence would be enough. Even Tsukiko might not know the difference if she'd simply never bothered to learn - which would certainly be very odd, but I suppose it wouldn't be the first time she was willfully blind to certain features of the undead.
Thank you for referencing my favorite episode. My favorite episode of what? Of anything, really.