It's really hard to explain, because I just know. Like, being male just feels different than being female, or some nonbinary genders. I guess it's somewhat similar to emotions - just like I know when I'm sad/happy/exited/etc I know when I'm male/female/other. It just feels very different. (And just like emotions can be fuzzy, my experience with gender often is as well, so sometimes it's less "I'm male" and more "I'm somewhat male but not completely, there's something else there as well, but I'm not sure what, definitely not female".) And yeah, I guess without society, I might not have words for my genders (for some I actually don't have words because they're completely independent from the gender binary), but the feeling would stay the same.