As good ol' Alice would say, Welcome to my nightmare!
First of all, make sure that you have the "Parallax on editor" set to yes. Click on any layer and look at its properties. After that, make sure that you are zoomed at 100%, you can achieve that by going into Constructs view tab, there is a button there that does just that.
Now, by panning around (middle click, or space bar) your scene and centering your point of view on different parts of your layout, you should have a good approximation of where is everything in relation with each other, taking into account the parallax offset. It's far from been accurate and the further away you are from 0,0 coordinates the more difficult it gets to arrange your scene.
Oh, how much I wish for a 2.5 d camera...