I know of a question (because I posted an answer to it), which was migrated to Chinese.SE:
日 and 月: Transition from pictographs to kanji
so I guess it's decided on a case-by-case basis, although it seems to be the only case.
It's clear that many questions about 漢字 can be answered by someone who knows Chinese. What's not clear, especially not to a Japanese learner, is whether a particular question about Japanese 漢字 can be answered without needing to know any Japanese. After all,
漢字 compounds may have a different meanings in Japanese and Chinese (a long discussion was sparked by this question about 方言),
stroke order may be different,
Japanese has its own names for the radicals,
there are 和製漢字,
When a question can be answered without any knowledge of Japanese, I agree that it should be migrated to the true experts on Chinese.SE. However, questions about Japanese 漢字 which require a knowledge of Japanese might better be left where they were asked. I'm assuming that answers on Chinese.SE will be formulated with a Chinese learner in mind. (There is no japanese on Chinese.SE, although on Japanese.SE there is chinese.)
Having many active users with an active knowledge of Chinese is a privilege we enjoy here on Japanese.SE.
Never mind my personal opinion, if any user on this site thinks that a particular question should better be asked on Chinese.SE, they can always leave a comment and (starting from
x reputation) cast a "migration" vote via the "close" link. That way it will be added to the close votes review queue on https://japanese.stackexchange.com/review. (That's how the above question was migrated.) I would interpret the first "migration" vote as a way of asking the community "do we want to migrate this question?". People against migration can always leave a comment explaining their reason (which is what often happens with "off-topic" close votes: "I disagree with the close vote for the following reason...").
I'm not sure what I suggested will work quite as smoothly as I just realized that I haven't tried to migrate anything from Japanese.SE to anything other than our meta (here). I just had a look at the menu in close > off-topic > This question belongs on another site... and migrating to Chinese.SE doesn't seem to be an option. What you can do is cast a custom close vote via close > off-topic > custom and say "I think this question belongs to Chinese.SE". This way it will get added to the review queue and other users can read your custom close vote and agree. Your custom close vote will leave a comment on the question and other close voters selecting your close reason will result in an automatic upvote on the automatic comment. Then a moderator (hopefully) sees that 5 people voted to close with the intention of migrating and migrate the question... Sorry for the confusion.