The Derailleur Hanger Alignment Gauge can help solve a multitude of derailleur shifting problems, most notably with indexed shifting systems. This tool is extremely steady in operation and simple to use.
The tool threads directly into the derailleur hanger on your bike. The tool has a gauge which marks the distance to the wheel. The gauge is then rotated with the wheel to the 3, 6, 9, and 12 o'clock positions, noting any differences in measurements. The alignment of the derailleur hanger is accomplished by applying pressure to the tool's arm. The results can be checked instantly. The DAG-2.2 is made of heavy gauge steel and is chrome plated for long shop life.
The DAG-2.2 features an upgraded, large inner shaft for added strength and was developed with tighter tolerances for improved accuracy. It has improved clearance and reach to fit more frame/hanger combos (including low clearance thru axle derailleur hangers).
I know a guy who bought one of these because he bends his hangers so frequently. Lol. Make sure that your rear wheel is as true as possible before using this tool. If you can't guarantee that, then move the wheel with the tool so that you're always measuring off of the same spot. Triple check all hanger for cracks and replace them if you see one. There are few things as catastrophic as a derailleur coming off mid-ride.