I mean, doing inverse projection stuff would be annoying, and of course there isn't enough information to reconstruct the 3D data exactly. (It's possible in theory, but what our brains use to determine depth is subtle angle stuff as well as shading/normal/texture data, basically, it's intractable [for us] to program it to solve a problem like this [even though software some day somewhere, maybe today, would be able to do it, it's probably not around yet and it's probably not freeware and it's probably difficult to set up and it's probably not very accurate :p ])
But, the idea of a projection is that the width on-screen is proportional to the distance. So, if the width at some point is 10 pixels, and the width of something which should be the same size at another point is 5 pixels wide, then the object is twice as far away. So you can actually hold a ruler up to your screen (or better yet use some tool, maybe GIMP, that allows you to draw a line and record the length of it), and then find ratios of sides to figure out ratios of distances.
Of course, this isn't completely accurate because the drawing may change in height or depth along the line that you're measuring, but if the lines you're measuring are mostly horizontal (so that the depth is approximately the same all along the line) and the lines you're measuring don't have much vertical rise, you should get pretty accurate results.
