They say when you die your brain has approximately 8 seconds of brain activity in which you go into a dream state. Now it is know that in a dream state 8 seconds can feel like a lifetime in which you live out your life. Now this would explain why we have deja vu and those people you've never met but look so familiar. So how are we to know if we are still alive or just reliving our previous life stuck in an infinite loop?
I came upon this article which has some pretty solid ideas about the science behind déjà-vus.
I think that you "relive your life" in a way that means you have already experienced everything that you can't re-experience; you're going back through all your memories and remembering the emotions you had once felt; you already know everything that happens in your life because you have lived it. The expression, "your life flashes before your eyes," means to me you remember and see your life go by in that short moment. You're no longer a living experiencing person in that life which has come to pass, but in that dream state you're a viewer, remembering what was.
This is an example of a "loaded question", with premises such as that we are "just reliving our previous life stuck in an infinite loop". Since we have no evidence to support this claim, it would be impossible to know if such a claim is true without this evidence.
On the question of our being "still alive", this depends on the definition of being "alive" in question. If one takes being "alive" to refer to one continuing their experience of life, then since we continually receive such a form of input, the existence of this input allows us to know that we are "still alive".