As a simple noun, 娘 does mean "(young) girl" as well as "daughter". Basically this usage is bookish, and it's not used outside established set phrases (e.g., 看板娘) in casual speech.
Sentence ending in 娘
Usage of を in 自らを豊作を司る
What purpose does ある serve in the sentence?
What does 看板親父 mean?
But that is not to say it's okay to address ...
The answer is nothing. Japanese does not have a special form for vocative other than nominative (not many languages have), and before the introduction of punctuation, you can just guess it from the context that it is an address. Even today, you hardly see commas and periods in speech balloons of manga, that means you still have to sometimes. And it is not ...
I should probably leave it to a native-speaker or someone with a better understanding to answer.
But, a quick answer is that when you address someone there's never a particle like は or が or を following. If you see ジョン without a particle or ジョンよ it's most likely direct address. But also, context often makes it very clear that someone is being addressed.