I think you'll find that many, many English-speakers think that Juliet is asking where Romeo is. Thus, knowing that she's asking why he's a Montague indicates

I think you'll find that many, many English-speakers think that Juliet is asking where Romeo is. Thus, knowing that she's asking why he's a Montague indicates that one is not one of those many people — it's a shibboleth for having had a minimally decent education.