"Ποιον να πιστέψει κανείς;"
Μετάφραση:Who to believe?
it's actually the subject of the sentence. it would translate to "one" or "anyone", as in "Who should one believe?" "Who should anyone believe?"
It's normal to use "κανείς" when you don't know the subject. For example, you think you're alone in a place and you suddenly hear a sound so you want to ask "Is there anyone there?". You would say "Είναι κανείς εκεί;". You come in the office for work and you ask your secretary "Did anyone call?" - "Τηλεφώνησε κανείς;"