Well... a lot of our experience of literature/fiction/life is not universal. Literary criticism is widely varied for a reason, even on works from the same country and language.
It's hard to get a handle on what you don't know, but you can approximate based on others. Think about depictions of Western culture that you've read in manga/manhua. Or times you've said out loud "that's totally cool but Christianity does ''not'' work that way" (at least that's what I do.) Then assume you are roughly as ignorant as they are. Ultimately, you're missing some of the culture not because of nihonjinron nonsense, but just because you aren't immersed in the culture constantly.
Another way to judge Japanese mythological reference knowledge is to read Urusei Yatsura and see how long you can go without asking why something happens or who an alien species represents in mythology. (The UY setting is basically Japanese mythology with a sci-fi shim.)
It's hard to get a handle on what you don't know, but you can approximate based on others. Think about depictions of Western culture that you've read in manga/manhua. Or times you've said out loud "that's totally cool but Christianity does ''not'' work that way" (at least that's what I do.) Then assume you are roughly as ignorant as they are. Ultimately, you're missing some of the culture not because of nihonjinron nonsense, but just because you aren't immersed in the culture constantly.
Another way to judge Japanese mythological reference knowledge is to read Urusei Yatsura and see how long you can go without asking why something happens or who an alien species represents in mythology. (The UY setting is basically Japanese mythology with a sci-fi shim.)