I'm pretty sure when you can remember it in as much detail as i can then you'd know you've been there before.
Addition:
Just for you, i went to wikipedia to back my statement up.
QUOTE
The "previous" experience is most frequently attributed to a dream, although in some cases there is a firm sense that the experience "genuinely happened" in the past.
Yes, that's what I was saying. Deja vu is when you think you remember having done the same thing you're doing now sometime in the past, it has nothing to do with precognition of future events.
Well lets put it this way. The person had a bad experiance and it was shifted into another concept but different scene
Personally, I prefer to avoid getting too freudian about these kinds of things. You have to be careful not to attach too much meaning to a dream, because it be just...not actually there.