Joined: Jan 2010
Posts: 9,835
Likes: 1,816
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
I agree that eyeball methods can be entirely sufficient.
As long as a known-straight derailer is on there, and a not-to-encrusted stack of cogs, one can sight down the cage pulleys from above and behind the bike by manually rotating the cage 90 degrees.
After such an evaluation, one can use a long adjustable wrench to correct both the twist and bend-in that may need to be corrected.
Positioning the wrench at the right angle will correct both simultaneously, usually by pulling outward.
It is very important to use a wrench that clamps fully over the threaded hole, so as not to distort it.
It is also importnt that the wheel be installed backwards in the frame (for wrench clearance) and that the QR lever be tightened (to prevent bending the axle dropouts out of alignment).
The job of bending the hanger back can take a few tries, flipping the wheel in the frame several times, when dealing with different hanger materials and with lack of wrench experience, but when the derailer finally sights straight, it is in fact straight enough for any indexing system at that point.