I've read the ppapers and lots of articles on the subject and still can't get how do I know where I land using Spectre Variante 1. I know I can read some byte i

I've read the ppapers and lots of articles on the subject and still can't get how do I know where I land using Spectre Variante 1. I know I can read some byte in Memory, but do I know where exactly it comes from? I know that I can increase the x value so that I can read other values, but until which point? Thanks