Well... It depends on what you mean by real now doesn't it. Consider this, does it have substance. I am not referring to physical substance, but abstract substance. Even if what you see is not physically there, it may be an abstract idea or character that is entirely substantial...
On a more believable and personal lv... I rarely get anything more than auditory hallucinations... The auditory hallucinations often sound like people I know, so I can know it isn't real if the person isn't around, and if they around I tend to answer the voice and look like an idiot. In those cases, it isn't real because it lacks substance. And it only really gets bad if I don't recognize the voice or it tells me to do something... often if it tells me to do something, I do it without trying to... it isn't so much of a command either.. it is like the voice gives me a piece of info and I act on it.
So for a pseudo-fictional example: I am in class doing work and I am waiting for my turn to leave my seat speak to the teacher about a recently finished rough draft.
The voice would say something like: (insert name) just finished talking to the teacher." and maybe I might feel a tap on the shoulder as the voice says it. Knowing that I am next in line after the person, I get up and walk across the room and either get in trouble or make a good excuse. In those cases, the voice is substantially real. In that particular example though it is sadly misleading.