I know it doesn't, but I wouldn't have thought to try it unless I read about it (probably somewhere here on BF) --

Put the bike in your favorite workstand (or upside down on the ground, whatever). Unscrew the derailleur from its hanger. Leave the hanger attached to the frame. Leave the rear wheel mounted on the bike.

Take an extra rear wheel, or at least a wheel that has exposed threads on the end of its axle, and screw it into the derailleur hanger -- yup, in the same hole that the derailleur used.

Carefully and thoroughly check that the spare wheel is just as vertical as the wheel mounted on the bike. Account for any wobbling movement of the spare wheel if you wiggle it by hand -- if it wobbles down, make sure that it wobbles upward the same amount past vertical (I'm terrible at describing this, but you'll get it), and figure that halfway between the wobbles is the "center". The wobble comes from the "sloppy fit" that Dan Burkhart just mentioned as I'm typing this post (I got the email notification just now ).

Carefully push and pull on parts of the spare wheel to bring it in vertical alignment with the wheel on the bike. Get a piece of tape measure or ruler to make sure that the centerline of one wheel is equidistant from the other at various points. Since their axles aren't on the same axis, you won't be able to simply measure tire-to-tire, or rim-to-rim, except at two points; otherwise, you'll be measuring from the center of one tire to underneath the rim of the other wheel, etc. Whatever parts you choose to measure, do it the same each time, like always rim-to-rim, or tire centerline-to-centerline, etc.

Stand back, look at it some more, then see if you need to tweak it again. Get the top and bottom aligned, then get the front and back aligned. Check other parts to be sure, too.

Unscrew the wheel from the hanger and re-mount the derailleur. You might have to readjust the screws or cable a bit to account for the newly realigned hanger.