Most of it is due to context, if I say:
"tomorrow brown is going to bath to meet carpenter" (notice that I have omitted the capital letters)
then "brown" is the subject of the verb so is a proper noun and not an adjective, "bath" is a place and not a washing tub or swimming pool and "carpenter" is the person that "brown" is going to meet and not, necessarily, a tradesman. With the capital letters it would be written:
"Tomorrow Brown is going to Bath to meet Carpenter"
Another clue is that proper nouns (including place names) very rarely have articles (a, an or the) in front of them. If the sentence was spoken as
"tomorrow brown is going to the bath to meet a carpenter"
then it would almost certainly mean the "brown" was going to the local swimming pool to meet a woodworker. Probably Brown would be something to do with the management of local leisure servives and there would be some work needed there. In this case the sentence would be written
"Tomorrow Brown is going to the bath to meet a carpenter."
Notice that "bath" and "carpenter" do not have capital letters but are preceded by articles.
Very occasionally proper nouns are preceded by an article but this happens very rarely and could confuse a native speaker as well. For instance "I'm going to see a Jackson today." would mean that I was going to meet a member of the Jackson family or a member of a firm or band called "Jacksons" or "The Jacksons" but this would only be used in context where everyone listening would know what I meant.
I hope this helps.