Marriage wrote After you die, its believed that you have 7 minutes of brain activity left inside you, and in the 7 minutes you experience your entire life over, in a kind of dream... Because Dream Time is stretched. So in this case, what if you're in that 7 minutes . How Do You know if your alive, or just reliving old memories.

You're almost right... it's 7 seconds. And yes, I've thought about this.
Think of this though. If you're within those 7 seconds, you would be in a dream-like state. Well, in dreams, you don't have a perception of time. So technically, that would mean that relative to everyone living, you would be dreaming for eternity when actually, it would only be 7 seconds from anyone who is watching you. This is what I would perceive to be as "afterlife".