As I've said before, general reference means that there is a type of reference source that is specifically designed to answer that type of question. The list is actually pretty limited: dictionary, thesaurus, and encyclopedia.
Note, for example, that there isn't a standard place to look up "is this grammatical" types of questions, so even the simplest question of that type is not, strictly speaking, general reference.
I don't know if we want to endorse particular dictionaries or encyclopedias. Perhaps we could do something like:
If there is a standard type of reference source specifically designed to answer your question, then it is off-topic here. For example, the definition of a word should be looked up in a dictionary.
Or we could get more specific:
If there is a standard type of reference source specifically designed to answer your question, then it is off-topic here. For the definition of a word, you should look in a dictionary; for synonyms and antonyms, you should look in a thesaurus; and for explanation of basic language concepts (grammar terms, parts of speech, etc.), you should look in an encyclopedia.