It means "And how can I know with whom I am speaking?"
The Spanish phrase, "Quien esta por ahi" means, "Who's around there?"in English
¿Quién eres? No puedo verte. Who are you? I can't see you.
"I'm in class. who's speaking?"
roughly: I am who you love, who waits for you and although I am not in your life, you'll always be in my life.
No puedo tolerar chicas quien son feas.
"De quien" means from who?; "De quien(?)-whose(?)";
"Con quien estas" translates to "who are you with" in English.
"Pregunto quien es" means:I'm asking, who is he?
"Quien vosotros buscáis" translates to "who are you looking for" in English.
"Hola quien eres" translates to "hello who are you" in English.
"Quien cumple años" in English means "who is having a birthday."
"Y esa quien esss" in English translates to "And who is that?"