I think a general guideline (borrowing somewhat from our site description) could be:
Will the addition of this question make the Internet a better place for students, teachers, and linguists wanting to find out about or discuss the finer details of the Japanese language?
In the case of your question, I would say to ask it here, even if it was on other sites for the following reasons:
There's a lot of noise and incorrect information on other sites such as forums and other Q&A sites, and there's value in asking it here to see how much the opinions of the users of JLU diverge or converge with those other sources.
The Stack Exchange voting system, while not infallible, can be a good indicator as to the accuracy of a given answer and is an advantage to asking here over many other sites. The comment system can also help with verifying information in a given answer.
There's some very dedicated and knowledgeable people here, and there are viewpoints which would not necessarily be presented on other sites. Even if it's the same or a similar question doesn't mean it will necessarily receive the same answers.
There's a chance the other website will go down, so it's good to have the information here as well (though as previously mentioned, there may be problems with copying and pasting answers directly.)
I would also say that even if your question was answered on a Japanese Q&A website (or other Japanese website such as the Japanese Wikipedia etc) but wasn't available in English, it's OK to also ask it here as not everybody using this site would necessarily be able to find, read or fully understand the information, and there may be value in making the information available to an English-speaking audience. It also may be of benefit to ask here to check whether you understood it correctly.
(Note if you didn't understand some of a given answer/explanation on another site, I think it's often a good idea to highlight what you didn't understand in the question, as it often results in more targeted answers and there's a good chance other people will have the same problem.)
However, I think there are some exceptions. When searching for (as a hypothetical question) "what is a Gairaigo?" on Google, it comes up with the English Wikipedia page for Gairaigo as the first result, which I think already has a lot of information and I'm not sure would be better served by the Q&A format of Stack Exchange websites (unless there was an aspect which wasn't already answered there that you wanted to know about.)
In my view there's also not much risk of Wikipedia disappearing from the Internet, so I'm not sure that point would apply in such cases (unless the article itself was removed from Wikipedia, which I think would be possible, though unlikely.)