I have had 2 visitation dreams. One was my deceased father, and the other was completely unrelated - benevolent strangers while I was travelling years before my father died. They are unlike any regular dream. Usual dreams are quite surreal, your point of view moves around, the locations are unlike the reality. Visitation dreams are very different, when you've had one, you know it was something different, something real.
My father visited me a year after he died. It was in my bedroom, I awoke on my bed and was aware of someone in the room. I turned around and was startled to find someone standing on my bed. I quickly jumped up, it was still dark, but I could see. My Dad was standing there smiling at me and saying "it's alright" over and over. He didn't speak with his mouth, but the message came through loud and clear in my mind.
When I woke in the morning, I knew something unique had happened. Everything about it was very real and the message in it was clear. No confusion about what it meant. In a regular dream, your friends and family might appear in it as characters, and often lots of weird surreal stuff is happening; but it's not the same as a visitation dream. When you have a visitation dream, you are left with no doubt that someone was trying to contact you with a message.
I know it's easy to write off dreams as just pure fantasy of the mind, but that's only if you've only ever had non-sensical regular dreams.