If it's at the beginning of the sentence, you can only distinguish from the context, or by intuition: probably it's a person talking about someone else, because who goes up to a stranger or their teacher to tell them that their clothing is usually black. You know what I mean? ;)
If it's in the middle of a sentence, you can distinguish them because the fromal you is always capitalized: Manchmal ist Ihre Kleidung bunt. (Sometimes your clothing is colorful.)
Meist and meistens are interchangeable, I believe. They both mean either mostly (like, nearly entirely something) OR most of the time / most often (in the temporal sense).
Meisten would show up with a definite article in plural to say "most (something / of something)."
Die meisten Lerner verwechseln das auch. Most learners mix that up too.
Or with the thing in genitive:
Die meisten meiner Freunde können kein Deutsch. Most of my friends can't speak German.
And that Die at the beginning is necessary although it seems strange in English.