Context? Dunno it's a toughie, but not as difficult as talking to a boz eyed person, which eye are you supposed to focus on? It's a diplomatic minefield.
If they start answering themselves you know they are talking to themselves...if they stop and look up for a response then you know they are talking to you.
Ask a question.
Its in the eyes if the pupil if faci.g you full on then they aretalking to you but if you see a lot of the white part of eye and they are speaking then sorry to say invisable friend has come along.