Two of my steel bikes, a De Rosa and an Eddy Merckx, have been crashed at some time in their lives – resulting in a bent derailleur hanger.
The last time I crashed the Merckx, I was going to have the shop straighten the derailleur hanger, but since this wasn’t the first time I’ve bent one, I thought I might as well invest in a Park Tool DAG-2.2 Derailleur Hanger Alignment Gauge
Using the tool is straight forward. One end of the tool threads into the the derailleur mounting hole. Using the tool as a lever allows the hanger to be bent back into alignment. There is an adjustable indicator that measures the run-out from the braking surface of the rim. When that distance is equal at four points (measured at 90 and 270 degrees for horizontal alignment and 0 and 180 degrees for vertical alignment), the derailleur hanger is in alignment.
Park Tool’s video on How to Align a Derailleur Hanger Using the DAG-2.2: