How do we know we are dead?

I was thinking about this last night when i was trying to get to sleep. I don't really believe this by the way, just thinking what if.
Say, for example you get hit on the head and you go unconscious for a few minutes. When you wake up again, maybe you died and you are living out your dream of how you expected your life to be.
This is in no way thought out well, like for example, what happens in this 'dream' when we die in it. i was wondering if anyone else had similar theories?