How to distinguish between the 'subject' and 'answer' in the development (modulatory) section of a fugue?
If the entries appear in isolation from each other, usually you can't label them as "subjects" and "answers" in any meaningful way. It is often more interesting to look at the overall key structure of the parts of the development section.
If there are a group of entries, or a stretto, and/or the tonal answer is significantly different from the subject, then it does make sense to identify the subjects and answers.
"A fugue" is a very ill-defined concept from an analysis point of view. On the one hand you have fugues like the D major from WTC book 2, where the first four notes of the subject occur something like 90 or 100 times in 50 bars of music. (Trying to count the exact number of entries, deciding in every case if the notes really are the start of the subject or just happenstance, is almost impossible.) On the other hand there are fugues by Handel (titled as such by the composer) where if you hadn't been told it was a fugue, there is so little structure to the piece you might not even realise that's what it was.