I have noticed various marks in example sentences to denote incorrect examples of English:
This is correct.
The former is left alone; the latter has an asterisk marking the sentence as a bad example ‐ something to avoid and not repeat.
Is this notation widely adopted? Are there other marks with similar purposes? I have also seen the following denoting a questionable case:
? This would have been maybe debated.
I am interested in the proper usage and formatting of these marks. How should they be spaced? Should they be placed before or after the sentence? If a particular word is in question, should that word get the mark or the entire sentence?