Background: I'm incorporating XKCD 386 (with the text modified) into a talk given in English with Japanese subtitles. I'm doing the cartoons side by side, and I'm wondering what would be the equivalent of underlining "wrong" in a western cartoon.
Dots added over (if writing horizontally) or to the right (if vertically) of each character.
Wikipedia Japan has a page detailing their use, as well as their variants:
(there are a couple other particular cases, such as when reporting an erratum, but I don't think you are particularly interested).
To conclude, you probably want to use the simple round black marker, like such (with slightly less distance between the lines, typically where the furigana would be):
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