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.
How it ends. Does it end in a random drop off or is the full sentence sound the same.
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.