I suggest the following convention:
- correct: (no mark). Usually, an example sentence is intended to be grammatically correct. Hence there's no need to mark them.
- questionable: '?' (
U+003F QUESTION MARK)
- incorrect: '*' (
As stated in the other posts, "*" and "?" are a well-established convention, used universally in linguistics. I've seen these two frequently, but have rarely seen "%" and "#" as described in sawa's post.
In particular, I don't think it's worth distinguishing "sometimes correct", "informant-dependent", "semantically strange" in our symbol convention. They are all subsumed by "questionable". The simplicity of using only 2 symbols is clearly advantageous. (If you need to distinguish, just do so in prose.)
"?" will be intuitive for everyone. Admittedly, "*" is not immediately obvious, but I think using the more widely established convention is still preferable. (Anyway, the "Japanese textbook" convention isn't universally obvious either.) Probably a FAQ entry on "What are all those asterisks?" is warranted.
The marks go in front of the example text, with no intervening space, and outside any quotes or brackets. For example:
- ?"Colorless green ideas sleep furiously." (syntactically correct, semantically self-contradictory)
- */je/ (phoneme sequence that does not occur in modern Japanese)
- "This is correct."
- *[nʲi.hon] (foreigners' mispronunciation of the moraic nasal)
- *⟨つずく⟩ (misspelling)