Part 1. Definition
A question that is free of any grammatical or logcal errors, should be labeled 'yes', otherwise it should be indicated as 'no'. A question is grammatically correct if all its entities i.e. nouns, verbs, adjectives, prepositions, pronouns, adverbs are at appropriate position. A question is logically correct if the semantic makes sense.
Part 2. Example
Sentence: Jack played basketball after school, after which he was very tired. 
Question: How long did Jack play basketball?
Answer: Yes.
Explanation: All entities are in appropriate position and the question makes sense, so its grammatically and logically correct.
Part 3. Exercise
Sentence: So really, that's what many of the people are doing now,"" he said. 
Question: How long will they be doing that?
Answer:
Yes.