If I were having a hallucination, would I know that it's not real?

Maybe. Most pts are aware but not all. I had a pt who kept her dog with her--if she heard a voice and her dog didn't look around she figured she was hallucinating!
Hallucinations. Hallucinations are psychotic processes. Psychosis means the patient is getting external stimuli from the environment through one or more of his or her five senses, that does not objectively exist. In other words there are auditory,visual, olfactory, gustatory, and tactile hallucinations that can be experienced by the patient. Auditory hallucinations, however, are the most common.
Usually not. The most likely cause of hallucinations involves brain activity without an actual real stimulus. Since we use our brain to understand what we see, hear, taste etc, it is often not possible to tell whether our brain received information from the environment or became activated through something akin to faulty wiring. Sometimes one can learn to tell if it is a hallucination but it can be difficult.